Musnad Ahmad 1153:428

Husayn narrated to us, Abu Uways narrated to us, he said: Muhammad ibn al-Munkadir narrated to us, from Sa’id ibn Jubayr, from ‘Aishah, that the Prophet, peace and blessings of Allah be upon him, said: "There is no person who has a prayer at night, and sleep overcomes him regarding it, except that Allah, the Almighty and Majestic, writes for him the reward of his prayer, and that sleep of his is a charity."