It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) fell asleep while in prostration until the sound of his breathing could be heard; then he ((peace and blessings be upon him)) stood up and prayed, and did not perform ablution.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10660