Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) would fall asleep in his prostration, and his sleep could be known only by the sound (of his breathing); then he (peace and blessings be upon him) would stand up in his prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9959