✅Answer: Please pay attention to the following narrations:

1 عَنِ الْحَسَنِ بْنِ رَاشِدٍ عَنْ أَبِي عَبْدِ اللَّهِ فِي حَدِيثٍ قَالَ : وَ لَا تَدَعْ صِيَامَ يَوْمِ سَبْعَةٍ وَ عِشْرِينَ مِنْ رَجَبٍ فَإِنَّهُ هُوَ الْيَوْمُ الَّذِي أُنْزِلَتْ فِيهِ النُّبُوَّةُ عَلَى مُحَمَّدٍ وَ ثَوَابُهُ مِثْلُ سِتِّينَ شَهْراً لَكُمْ.

📚Al-Faqih, Vol. 2, p. 90

📚Thawab al-A'mal, p. 99, Hadith 1

📚Al-Kafi, Vol. 4, p. 148

📚Al-Tahdhib, Vol. 4, p. 305

Imam Sadiq (peace be upon him) said in a hadith: 'Do not abandon the fast of the 27th of Rajab, for it is the day that prophethood was revealed to Muhammad (peace be upon him and his family), and its reward for you is like that of 60 months!'

2 عَنْ عَبْدِ اللَّهِ بْنِ طَلْحَةَ عَنِ الصَّادِقِ جَعْفَرِ بْنِ مُحَمَّدٍ قَالَ: مَنْ صَامَ يَوْمَ سَبْعَةٍ وَ عِشْرِينَ مِنْ رَجَبٍ كَتَبَ اللَّهُ لَهُ صِيَامَ سَبْعِينَ سَنَةً.

📚Amali al-Saduq, p. 470, Hadith 7

Imam Sadiq (peace be upon him) said: 'Whoever fasts on the 27th of Rajab, Allah will write for him the fast of seventy years!'

3 عَنْ أَبَانِ بْنِ عُثْمَانَ عَنْ كَثِيرٍ النَّوَّاءِ عَنْ أَبِي عَبْدِ اللَّه فِي حَدِيثٍ قَالَ: وَ فِي الْيَوْمِ السَّابِعِ وَ الْعِشْرِينَ مِنْهُ يَعْنِي مِنْ رَجَبٍ- نَزَلَتِ النُّبُوَّةُ عَلَى رَسُولِ اللَّهِ مَنْ صَامَ هَذَا الْيَوْمَ كَانَ ثَوَابُهُ ثَوَابَ مَنْ صَامَ سِتِّينَ شَهْراً.

📚Amali al-Tusi, Vol. 1, p. 43

Imam Sadiq (peace be upon him) said in a hadith: 'On the twenty-seventh of that month, meaning the month of Rajab, prophethood was revealed to Muhammad (peace be upon him and his family). Whoever fasts on this day, his reward is the reward of one who fasts for sixty months!'

4 عَنْ سَهْلِ بْنِ زِيَادٍ عَنْ بَعْضِ أَصْحَابِنَا عَنْ أَبِي الْحَسَنِ الْأَوَّلِ قَالَ: بَعَثَ اللَّهُ عَزَّ وَ جَلَّ مُحَمَّداً رَحْمَةً لِلْعَالَمِينَ فِي سَبْعٍ وَ عِشْرِينَ مِنْ رَجَبٍ فَمَنْ صَامَ ذَلِكَ الْيَوْمَ كَتَبَ اللَّهُ لَهُ صِيَامَ سِتِّينَ شَهْراً الْحَدِيثَ.

📚Al-Kafi, Vol. 4, p. 149

Imam Kadhim (peace be upon him) said: 'On the 27th of Rajab, Allah Almighty sent Muhammad (peace be upon him and his family) as a mercy to the worlds. So whoever fasts on that day, Allah will write for him the fast of 60 months...'

5 عَنْ أَبِي إِسْحَاقَ بْنِ عَبْدِ اللَّهِ عَنْ أَبِي الْحَسَنِ عَلِيِّ بْنِ مُحَمَّدٍ أَنَّهُ قَالَ لَهُ الْأَيَّامُ الَّتِي يُصَامُ فِيهِنَّ أَرْبَعَةٌ أَوَّلُهُنَّ يَوْمُ السَّابِعِ وَ الْعِشْرِينَ مِنْ رَجَبٍ يَوْمَ بَعَثَ اللَّهُ مُحَمَّداً إِلَى خَلْقِهِ رَحْمَةً لِلْعَالَمِينَ الْحَدِيثَ.

📚Al-Tahdhib, Vol. 4, p. 305

In a hadith, Imam Hadi (peace be upon him) said to Abu Ishaq: 'There are four days on which fasting is observed: the first of them is the twenty-seventh of Rajab, the day on which Allah sent Muhammad (peace be upon him and his family) to His creation as a mercy to the worlds!'

6 عَنِ الرَّيَّانِ بْنِ الصَّلْتِ قَالَ: صَامَ أَبُو جَعْفَرٍ الثَّانِي لَمَّا كَانَ بِبَغْدَادَ صَامَ يَوْمَ النِّصْفِ مِنْ رَجَبٍ وَ يَوْمَ السَّابِعِ وَ الْعِشْرِينَ مِنْهُ وَ صَامَ مَعَهُ جَمِيعُ حَشَمِهِ الْحَدِيثَ.

📚Misbah al-Mutahajjid, p. 750

Rayyan ibn Salt said: 'When Imam Jawad (peace be upon him) was in Baghdad, he fasted on the 15th of Rajab and the 27th of Rajab, and all of his family and household fasted with him!'

✅It is observed that in Shi'a narrations, the twenty-seventh of the month of Rajab is introduced as the day of the Prophet's mission (bi'tha) of the Messenger of Allah (peace be upon him and his family), and a great reward has been designated for fasting on that day.

اللَّهُمَّ فَصَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ