Sahih Bukhari Volume 1, Book 12, Hadith Number 737

Narated By Jabir bin Samura : ‘Umar said to Sa’d, “The people complained against you in everything, even in prayer.” Sa’d replied, “Really I used to prolong the first two Rakat and shorten the last two and I will never shorten the prayer in which I follow Allah’s Apostle.” ‘Umar said, “You are telling the truth and that is what I think a tout you.”