Yahya related to me from Malik from Zayd ibn Aslam that Umar ibnal-Khattab said, "If you fall asleep Iying down you must do wudu (before you pray) . " (DE)
USC-MSA web (English) reference: Book 2, Hadith 10 (DE)
Arabic reference (DE): Book 2, Hadith 39 (DE)