Musnad Ahmad 1153:1446

'Abd al-Rahman narrated to us, from Malik, from Muhammad ibn al-Munkadir, from Sa'id ibn Jubayr, from a man, from 'Aishah, from the Prophet, peace and blessings of Allah be upon him, who said: "There is no person who has a prayer at night that sleep overcomes him, except that his sleep is a charity for him, and the reward of his prayer is written for him."