Muwatta Imam Malik (Riwayat Ibn al-Qasim)

Saeed bin Jubair (may Allah have mercy on him) narrates from his preferred person (Aswad bin Yazid, may Allah have mercy on him) that Umm al-Mu'mineen, Sayyidah Aisha (may Allah be pleased with her), informed him that the Messenger of Allah (peace and blessings be upon him) said: "Whoever regularly prays (voluntary) prayers at night, and then sleep overcomes him (and he sleeps and is unable to pray), the reward of the prayer is written for him, and his sleep becomes a charity for him from Allah."