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) . " (ID)
USC-MSA web (English) reference: Book 2, Hadith 10 (ID)
Arabic reference (ID): Book 2, Hadith 39 (ID)