Musannaf Ibn Abi Shaybah

Sufyan was asked about a man who was present at home at the time of the sun's decline, then set out on a journey; how should he perform the prayer? He replied that if he prays during the time of Zuhr, he should perform two rak'ahs. Hasan bin Salih says that when the sun declines here, then on the journey he should perform four rak'ahs. Sufyan, regarding a traveler who joins the prayer with a resident and prays one rak'ah with him, then sees something and talks, and in the meantime the imam completes the prayer, says that the traveler should repeat two rak'ahs and then return to the original ruling. Hasan says that he should perform four rak'ahs because he has made four rak'ahs obligatory upon himself.