'A'isha reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When one of you dozes off while he is praying, he should go and lie down until he is no longer sleepy. If someone prays when he is drowsy, he may not know whether he is asking for forgiveness or asking for something bad for himself."

Riyadh-as-Saliheen by Imam Al-Nawawi, volume 1, hadith number 147 - Sunni Hadith