It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) slept until he began to snore, then he said: "Ablution (wudu) is obligatory on one who sleeps lying down."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7917