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
Suwayd bin Ghafalah narrates from Sayyiduna Abu al-Darda (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever comes to his bed intending to stand in prayer at night, but sleep overcomes him until morning, the reward of what he intended is recorded for him, and his sleep is a charity to him from Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4724