Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "Ablution is not obligatory upon one who falls asleep while in prostration, until he lies down, for when he lies down his joints become relaxed."
Ahmad and Abu Ya'la recorded it, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1286