Abu Dawud Book 2, Hadith Number 1305.
Chapter : Dozing during prayer.
Narated By ‘Aisha, wife of the Prophet (PBUH) : The Prophet (PBUH) said: When one of you dozes in prayer he shold sleep till his sleep is gone, for when one of you prays while he is dozing, perhaps he will curse himself if he begs pardon of Allah.