Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: I saw the Prophet (peace and blessings be upon him) pray two rak'ahs before Fajr, then he fell asleep until he snored and his breathing could be heard, while he (peace and blessings be upon him) was in prostration or sitting. Then he got up and prayed. I said: O Messenger of Allah! You had fallen asleep! He (peace and blessings be upon him) said: "Ablution (wudu) is obligatory only upon one who sleeps lying down, for when he lies down his joints relax."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12693