Sayyidah Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "Any servant who stands in prayer at night and is then overcome by sleep, Allah Almighty records for him the reward of his prayer, and his sleep is a charity for him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4723