Musannaf Ibn Abi Shayba 3:6252

Abu Bakr narrated to us, he said: I heard Waki’ saying: Sufyan said regarding a man for whom the sun had passed its zenith while he was in the city, then he went out on a journey: How should he pray? He said: If he is within the time of Dhuhr, he prays two rak'ahs. And Hasan ibn Salih said: If the sun has passed its zenith here, he prays four [rak'ahs] while on a journey. He said: And Sufyan said regarding a traveler who entered [a prayer] with a resident and prayed one rak'ah with him, then he saw something and spoke, and the Imam prayed [the rest of the prayer]. He said: The traveler repeats two rak'ahs, then he returns to the original state he was upon. And Hasan ibn Salih said: He prays four, because he had made it obligatory upon himself.