Short answer
No authentic hadith sets a special fast for Rajab or for its 27th day. Rajab is one of the sacred months, and fasting in it is good like fasting in any month, so the Hanafi texts list it among the desirable fasts. But do not believe that a particular Rajab fast carries a fixed reward, as the reports about that are weak or fabricated.
Voluntary fasting in Rajab is mustahabb as part of the sacred months. Singling out the 27th of Rajab with the belief that it is a sunnah fast with a set reward has no sound basis; Hanafi scholars such as Mufti Taqi Usmani advise against treating it as one.
Other madhhabs
- Hanbali
- Fasting the whole of Rajab alone is makruh; break it with a day or more, or fast another month with it.
Explanation
Rajab is the seventh month of the Hijri calendar and one of the four sacred months. People often hear of special Rajab fasts, especially on the 27th, the night many associate with the Isra and Miraj. Here is what is sound and what is not.
What is sound. Uthman ibn Hakim asked Sa'id ibn Jubayr about fasting in Rajab, while they were in Rajab. Sa'id replied with Ibn Abbas's description of the Prophet ﷺ: he would fast until people said he would not stop, and leave fasting until they said he would not fast. In other words, Rajab has no special fast, and the normal sunnah of fasting applies in it as in any month. Since it is a sacred month, the Hanafi texts count fasting in it among the desirable fasts.
What is not sound. Reports that whoever fasts the 27th of Rajab gets the reward of sixty months, or that a day of Rajab fasting equals a month, are weak or fabricated by the judgement of the hadith scholars. The date of the Isra and Miraj itself is uncertain. So do not fast the 27th believing it is a sunnah with a known reward.
- Fast Mondays, Thursdays and the white days in Rajab as usual.
- If you fast the 27th because it happens to be your Monday or Thursday, that is fine.
- Use Rajab to start preparing for Sha'ban and Ramadan.
See Isra and Miraj, the four sacred months and fasting in Sha'ban.
The evidence
Quran, Surah At-Tawba 9:36
إِنَّ عِدَّةَ ٱلشُّهُورِ عِندَ ٱللَّهِ ٱثْنَا عَشَرَ شَهْرًۭا فِى كِتَٰبِ ٱللَّهِ يَوْمَ خَلَقَ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضَ مِنْهَآ أَرْبَعَةٌ حُرُمٌۭ ۚ ذَٰلِكَ ٱلدِّينُ ٱلْقَيِّمُ ۚ فَلَا تَظْلِمُوا۟ فِيهِنَّ أَنفُسَكُمْ ۚ وَقَٰتِلُوا۟ ٱلْمُشْرِكِينَ كَآفَّةًۭ كَمَا يُقَٰتِلُونَكُمْ كَآفَّةًۭ ۚ وَٱعْلَمُوٓا۟ أَنَّ ٱللَّهَ مَعَ ٱلْمُتَّقِينَ ﴿٣٦﴾
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him].
Rajab is one of the four sacred months, which have special respect.
Sahih Muslim 1157
Sahih... سألت سعيد بن جبير عن صوم رجب ...
"I asked Sa'id ibn Jubayr about fasting in Rajab, and he replied with the Prophet's practice of fasting for a while and then not fasting for a while."
Rajab has no fixed special fast; normal voluntary fasting applies in it.
Where the ruling is recorded
- al-Fatawa al-Hindiyya, Kitab al-Sawm, chapter on what is desirable and disliked in fasting. Fasting in Rajab is listed among the desirable fasts.
- al-Mughni (Ibn Qudama), Kitab al-Siyam, chapter on voluntary fasting. The Hanbali dislike of fasting the whole of Rajab alone.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.