Sahih Muslim Book 4, Hadith Number 1718.

Chapter : Excellence of an act (i.e. observe of the night prayer, etc.) done constantly.

‘Aisha reported Allah’s Apostle (may peace be upon him) as saying: When anyone amongst you dozes in prayer, he should sleep, till sleep is gone, for when one of you prays while dozing he does not know whether he may be asking pardon or vilifying himself.