Maliks Muwatta Book 7, Hadith Number 1.

Yahya related to me from Malik from Muhammad ibn al-Munkadir from Said ibn al-Jubayr that a man who has approval (as a relater of hadith), told him that A’isha, the wife of the Prophet, may Allah bless him and grant him peace, said, “If a man prays in the night and sleep overcomes him during it, Allah writes for him the reward of his prayer, and his sleep is sadaqa for him.”