Reference: Fataawa as-Siyaam libni Jibreen p136 no.161
Question: Is standing the night in prayer done only in Ramadhaan or on every night of the year? And from which time does it begin and which time does it end? And is the standing only the prayer or is it [both] the prayer and reciting the Qur.aan?
Response: Standing the night in prayer and Tahajjud are a Sunnah and most virtuous, and the Prophet ﷺ and his sahaabah used to safeguard it, as in the statement of Allaah:
, and it is not specific to the month of Ramadhaan.
It's time is from between the 'Ishaa. prayer until the Fajr prayer, however, the prayer in the last part of the night is superior, and if it was performed in the middle [of the night] then it is rewarded, whilst it is preferable to do so during the last half of the night, and Allaah [alone] has the Complete knowledge [of all affairs].
