It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both): "Ablution is obligatory upon everyone who sleeps, except one whose head merely nods (from drowsiness); (ablution is not due on him on account of such sleep)."
(b) A group narrated it from Yazid bin Abi Ziyad in mawquf form, and there is also a marfu' narration, but its being marfu' is not established.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 583