Musnad Ahmad 1153:803

Abu Ahmad narrated to us, he said: Abdullah bin Habib narrated to us, from Habib bin Abi Thabit, from Ata bin Yasar, who said: A man came and spoke ill of Ali and Ammar, may Allah be pleased with them both, in the presence of Aisha. She said: "As for Ali, I will not say anything to you regarding him. But as for Ammar, I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'He is not given a choice between two matters, except that he chooses the most guided of the two.'"