Ruling on one who falls asleep whilst praying
Question

                Sometimes drowsiness overwhelms me during Fajr prayer and I do not know what I am saying or what I am doing, such as reciting Qur’aan and “al-tahiyyaatu…” But it is not deep sleep in which there is the fear of wudoo' being invalidated. What should I do?.
Praise be to Allah.
      So long as you are still conscious and understand the 
      prayer and are following the imam, then your prayer is valid and the basic 
      principle is that you have done what the imam does. This is the basic 
      principle and this is what you have apparently done with the imam, unless 
      you know something to the contrary. If you know that you did not recite 
      al-Faatihah, you should recite it and then say the salaam after that; if 
      you know that you did not recite the tahiyyaat with the imam, recite it 
      and then say the salaam. If you know that you omitted al-Faatihah in one 
      of the rak’ahs, then the matter is broad in scope, because the imam will 
      carry that responsibility for you in the event that you forgot it or did 
      not know that you should recite it; or if you do not know what you did, 
      the imam will carry that responsibility for you, in sha Allah. End quote. 

    Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).