(a) It is narrated from Abu Awn that he said: I heard Jabir bin Samurah (may Allah be pleased with him) say that Sayyiduna Umar (may Allah be pleased with him) said to Sa'd (may Allah be pleased with him): The people of Kufah have complained about you in everything, even about the prayer. He said: As for me, I lengthen the first two rak'ahs and shorten the last two, and I took this from the prayer of the Messenger of Allah (peace and blessings be upon him); I do not fall short in it in any way. Umar al-Faruq (may Allah be pleased with him) said: You have spoken the truth; this is what was expected of you.
(b) In the hadith of Shababah it is: I lead them in the first two rak'ahs at length and the other two briefly. At its end it is that he (may Allah be pleased with him) said: O Abu Ishaq, this is what I thought of you.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2482