Resurrected from the now defunct “Islamic Law, Etc.” blog, here is a post I made a long time ago about the permissibility of saying “Ramadan Mubarak.”
In the name of Allah, the Beneficent, the Merciful.
Every year Muslims all over the world wait in eagerness for the coming of the month of Ramadan. Ramadan, the month in which every night Allah has designated people to be freed from the hellfire, the month in which there is a night better than one thousand months, whoever fasts it with faith and reflection then all of his past sins will be forgiven.
Because of the status of this month and its importance, many of us greet each other in excitement with phrases such as “Ramadan Mubaarak” , “Ramadan Kareem”, “Kullu ‘aam wa antum bi khair” anticipating the great blessings of this month and wishing them for others.
Yet these phrases and greetings, even though we use them frequently, do they have a basis in our religion? Meaning: is there a precedent which has been set for such greetings?
When discussing issues pertaining to custom and common practice, it is important to remember that the base ruling for all customs is that they are permissible as long as they do not contradict an established prohibition or contain a hazard to ones religion.
In the context of our discussion, not only is there no established prohibition for this type of greeting, but it is also void of any harm. Thus such a greeting in fact contains benefit, being that it is a manner of supplication by Muslims for each other, as well as a way to unite thehearts. Therefore this type of greeting is permissible. 
In addition to this general principle, several scholars of the past have mentioned narrations which support this issue specifically, such as the hadeeth narrated by Imam Ahmad in his Musnad:
On the authority of Abu Hurairah (may Allah be pleased with him) that the Messenger ofAllah (Peace and blessings upon him) said whilst giving glad tidings to his companions,
“The month of Ramadan has come to you, a blessed month;
Allah has obligated you to fast in it, in it the gates of paradise have been
opened and the gates of hell have been closed, and in it the Shayaateen have
been shackled; in it is a night better than one thousand months, whoever was
prevented its good has truly been prevented.” 
Commenting on this hadeeth, Ibn Rajab Al Hanbali said:
“Some scholars have said ‘This hadeeth is a fundamental evidence for [the
permissibility of] people greeting each other with [the commencement] of Ramadan…So how can a believer not be given glad tidings when the doors of paradise have been opened? How can the sinner not be given glad tidings upon the closing of the gates of Hell? How can an intelligent person not be given the glad tidings of a time in which the Shayaateen are shackled…?” 
Although this hadeeth has a Saheeh (authentic) chain of narration up until the narrator Abu Qilaabah, who was from the generation following the Sahabah, he did not hear anything from Abu Hurairah, the narrator of this hadeeth. This then makes it a Mursal narration; Mursal narrations are slightly weak narrations because of the unknown narrator between the Taabi’ee and the Companion.
Even with this slight weakness, it does however have some basis in what was narrated by both Bukhari and Muslim, the wording of Bukhari being:
“When the month of Ramadan comes to you, the gates of
paradise are opened, and the gates of hell are closed, and in it the Shayaateen are shackled.” 
The remainder of the wording of Ahmad can be found in Ibn Maajah , yet with a weak chain.
So although these narrations (i.e. from Ahmad and Ibn Majah) have some weakness, the narration of Imam Ahmad is Saheeh Mursal, and even though Mursal narrations are considered weak, weak narrations are acceptable in acts of virtue (Fadaail al ‘Amaal). This opinion was held by many scholars of the past, from them the likes of Imam Ahmad ibn Hanbal (Died 241), Ibn Mahdi (Died 198), and Ibn Al Mubaarak (Died 181).
Yet even with this, narrations with slight weakness were not left unrestricted. Al Haafiz Ibn Hajr placed conditions for the acceptance of these types of narrations. All four Imams have accepted Mursal narrations, with certain conditions, in legislative rulings also. 
Therefore one should be careful not to accept just any narration concerning virtue because it seems acceptable to the self. Instead the acceptance of such narrations should be dictated by scholars who are well grounded in the science of hadeeth, because of the grave danger of lying in the Messenger of Allah’s (peace be upon him) name.
All of what has preceded supports the general principle mentioned in the beginning of this article, that this action is a custom and has not been contradicted by a known prohibition nor does it contain any harm. On the contrary, based on these previous narrations, we can say that there is some basis for greeting one another at the commencement of Ramadan, even if thesenarrations have slight weaknesses. Therefore, the ruling remains that such an act is permissible.
In addition to what was just mentioned, it is suitable here to mention what Imam Ibn Al Qayyim has said while commenting of the hadeeth of Ka’b ibn Maalik may Allah be pleased with him , when he and his companions were given the glad tidings of their repentance being accepted. He says :
“…in it (i.e. the story of Ka’b) is evidence that it is recommended to
congratulate the one who has had a religious blessing renewed for him, stand for him when he approaches and shake his hand, this is a recommended sunnah. It is also permissible [to be done] for the one who has had a worldly blessing renewed…” 
No doubt that reaching the month of Ramadan is a religious blessing, it therefore is more deserving to be congratulated with than other blessings which occur through out the year, being that it is not only reoccurring but also general for all Muslims.
This is supported by the narrations from the Salaf that they would supplicate to Allah
“Oh Allah, deliver me to Ramadan, and deliver it to me. And receive from me
in acceptance.” 
Imam Al Suyuti relates from Al Haafiz Ibn Hajr that he said
“ Evidence for the general permissibility of congratulations
when a blessing takes place, or an evil is averted is
the permissibility of Sujood Al Shukr (Prostration of thanks)
and of giving ones’ condolences; perhaps also the narration
of Ka’b ibn Maalik in the two Sahihs…” 
Several contemporary scholars have commented on the issue, such as Shaikh Abdul Aziz Ibn Baz. When asked about congratulating one another at the commencement of Ramadan he said, “[It is] Good”. Similarly when Shaikh Muhammad ibn Saalih Al Uthaimeen was asked about it he said it was, “Very good”.
The great scholar of tafseer, Muhammad Al Ameen Al Shinqeeti, commented that because he does not know of any established precedent for this type of greeting, that sticking to what has been narrated would be best, but if one was to congratulate you on the commencement of Ramadan then there is no harm in replying to him out of “returning the greeting with better then it” and because “All supplication is good and blessed, yet one should not adhere toa specific phrase or greeting”. 
To conclude, it seems, and Allah knows best, that the issue at hand is a permissible custom supported by the general permissibility of customs in Islam.
In addition there is set precedence in the matter by way of narration as well as the statements of the scholars to underline this ruling.
May Allah bless all of us to gain His pleasure during Ramadan, be written among those freed from the Hellfire, and have all past sins forgiven. Ameen.
و صلى الله على نبينا محمد
و على آله و صحبه
 [Al Sa’di, Almajmoo’ah al kaamliah 8/34].
 [Musnad Ahmad 2/230, 385, 425, see also Al Nisaa’i 4/126]
 [Ibn rajab, lataaif al ma’aarif P.281]
 [Bukhari, Al Jami Al Saheeh 1899]
 [Ibn Maajah, Al Sunan1644]
 [see Al Suyooti, Tadreeb Al Raawi P.222 and pg 350-351, also Al Qaasimi,Qawaaid Al Tahdeeth 1/116].
 [see Al Suyooti, Tadreeb Al Raawi P.223-224]
 [narrated in Bukhari and Muslim]
 [Zaad al Ma’aad 3/512]
 [Ibn Rajab, Lataaif al Ma’aarif P.280]
 [Al Suyooti, wusool Al Tahaani, found in Al Haawi 1/83. See Also IbnHajar, Juz’ fi al tahniah fi al ‘Ayaad; as well as Al Mawsooah Al fiqhiyyahAl Kuwaitiyyah 14/99-100].
 [Al Muqbil, Hukm Al Tahni’ah bi dukhool Ramadan]