Abu Khalid al-Dalani أبو خالد الدالاني الأسدي · The Book on Purification

he saw the Prophet sleeping, while in the prostration position, until he snored or snorted. Then he stoodup to pray. So I said: "O Messenger of Allah! You were sleeping?" He said: "Wudu is not required except for sleeping while reclining. For when one reclines, joints relax."