Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 914
Jabir b. Samura reported: 'Umar said to Sa'd: They complain against you in every matter, even in prayer. He (Sa'd) said: I prolong (standing) in the first two (rak'ahs) and shorten it in the last two, and I make no negligence in following the prayer of the Messenger of Allah (may peace be upon him). He ('Umar) remarked: This is what is expected of you, or, that is what I deemed of you.