From Sahih Muslim (Hadith 1584):
Jubair b. Nufair reported:I went along with Shurahbil b. al-Simt to a village which was situated at a distance of seventeen or eighteen miles, and he said only two rak'ahs of prayer. I said to him (about it) and he said: I saw 'Umar observing two rak'ahs at Dhu'l-Hulaifa and I (too) said to him (about it) and he said: I am doing the same as I saw the Messenger of Allah (ﷺ) doing
Reference: Muslim 1584