Sunan Abu Dawood, Chapter: Purification (Kitab Al-Taharah)

The Apostle of Allah (peace_be_upon_him) used to prostrate and sleep (in prostration) and produce puffing sounds (during sleep). Then he would stand and pray and would not perform ablution. I said to him: you prayed but did not perform ablution though you slept (in prostration). He replied: Ablution is necessary for one who sleeps while he is lying down. Uthman and Hannad added: For when he lies down, his joints are relaxed.