Musannaf Ibn Abi Shayba 1:1036

Jarir narrated to us, from Ash'ath, from Ja'far, from Sa'id ibn Jubayr, who said: Ibn Abbas was on a journey with people from the companions of the Messenger of Allah, peace and blessings of Allah be upon him, among whom was Ammar ibn Yasir. They used to put him forward to lead them in prayer due to his kinship with the Messenger of Allah, peace and blessings of Allah be upon him. He led them in prayer one day, then he turned to them and laughed. He informed them that he had had intercourse with a Roman slave girl of his, and he had led them in prayer while he was in a state of major ritual impurity (junub), so he performed tayammum.