Mukhtasar Bukhari

Narrated from Ummul Momineen Aisha Siddiqa (may Allah be pleased with her) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When any one of you becomes drowsy while praying, he should (break the prayer and) go to sleep until his sleep goes away. Because if any one of you prays in a state of sleepiness, he does not know (what he is saying); perhaps he intends to seek forgiveness but (unknowingly) he may curse himself."