Yahya related to me from Malik from Zayd ibn Aslam that Umar ibn al-Khattab said, "If you fall asleep Iying down you must do wudu ( before you pray) . "
Muwatta Malik Book 2, Hadith 8
← Previous Hadith Yahya related to me from Malik from Abu'zZinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "When you wake up from sleep to pray, wash your hands before you put them in the wudu water, for you do not know where your hands have spent the night."

Muwatta Malik Book 2, Hadith 7 Next Hadith → Yahya related to me from Malik from Zayd ibn Aslam that the ayat "You who believe! When you rise for prayer wash your faces, and your arms to the elbows, and wipe over your heads and your feet up to the ankles," refers to rising from bed, meaning sleep.

Yahya said that Malik said, "The situation with us is that one does not have to do wudu for a nose-bleed, or for blood, or for pus issuing from the body. One only has to do wudu for impurities which issue from the genitals or the anus, or for sleep."

Muwatta Malik Book 2, Hadith 9