Sahih Muslim Book 6, Hadith 15
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.
I asked Anas b. Malik about shortening of prayer. He said: When the Messenger of' Allah (ﷺ) had covered a distance of three miles or three farsakh (Shu'ba, one of the narrators, had some doubt about it) he observed two rak'ahs.
This hadith has been transmitted by Shu'ba with the same chain of narrators and it is narrated from Simt, and the name of Shurahbil has not been mentioned, and he said that he had gone to a place called Dumin, situated at a distance of eighteen miles from Hims.