Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1718
'A'isha 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.