Musannaf Ibn Abi Shayba 3:2474

Ali ibn Mushir narrated to us, from Isma'il ibn Abi Khalid, who said: I heard al-Sha'bi saying: If a man enters into the obligatory prayer, then the iqamah (call to commence prayer) comes upon him suddenly, he should cut it off, and it shall be for him a voluntary prayer, and he should enter into the obligatory prayer.