Sahih Muslim Book 4, Hadith Number 1600.

Chapter : The virtue of Sunan prayers before and after the Fard Rak’ahs and their number.

‘Abdullah b. ‘Amr reported: It was narrated to me that the Messenger of Allah (may peace be upon him) had said: The prayer observed by a person sitting is half of the prayer. I came to him (may peace be upon him) and found him praying in a sitting position. I placed my hand on his head. He said: O ‘Abdullah b. ‘Amr, what is the matter with you? I said: Messenger of Allah, it has been narrated to me that you said: The prayer of a man in a sitting position is half of the prayer, whereas you are observing prayer sitting. He (the Holy Prophet) said: Yes, it is so, but I am not like anyone amongst you.