Sahih Muslim Book 4, Hadith Number 1629.

Chapter : Permissibility of observing Nafl (voluntary prayer) standing or sitting and observing some part of it standing or sitting.

‘Umar b. Khattab reported Allah’s Messenger (may peace be upon him) as saying: Should anyone fall asleep and fail to recite his portion of the Qur’an, or a part of it, if he recites it between the dawn prayer and the noon prayer, it will be recorded for him as though he had recited it during the night.