Musannaf Ibn Abi Shayba 3:2477

Abu Bakr narrated to us, he said: Abu Bakr ibn Ayyash narrated to us, from Mughirah, from al-Sha’bi, who said: "If a man is standing, praying, and he hears the iqamah, he should cut [his prayer] short." And Ibrahim said: "He should add another [rak'ah] to it and not cut it short."