Rāyyan bin Shabeeb (the maternal uncle of Mo'tasim), says that I went to meet Imām Ali ar Rezā (alayhi salām) on the First day of the month of Muharram
Imām Alī ibn Mūsā al-Rezā (alayhi salām) asked:
“O son of Shabeeb! Are you in the state of fasting today”?
I (Rāyyan bin Shabeeb) replied in the negative. Imām (alayhi salām) continued,
“This is the day when Prophet Zakariyyah (alayhi salām) prayed to his Lord thus:
“Lord grant me from unto Thee a good offspring, Verily Thou art the Hearer of Prayers” (Surah al-Ali Imran, 3:38).
Then Allāh accepted his Prayers and commanded His Angels to go and give him glad tidings regarding the birth of his son Prophet Yahyā (alayhi salām). The Angels came and called out to him while he was engaged in Prayers in the niche. Hence the one who fasts on this day and asks for his desires from Allāh, his Prayer will be answered as was of Zakariyāh (alayhi salām).”