It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him): "Whoever has truly fallen asleep, ablution has become obligatory upon him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 584
Hadith Gradingمحدثین:ضعيف أخرجه علي بن الجعد 1452