Musannaf Ibn Abi Shayba 35:116

Ibn ‘Ulayyah narrated to us, from Khalid, from Abu Qilabah, from Abu al-Muhallab, from ‘Imran ibn Husayn, that the Prophet (peace and blessings of Allah be upon him) prayed the ‘Asr prayer, then he performed the taslim after three rak’ahs, then he departed. A man called al-Khirbaq stood up to him and said, "O Messenger of Allah, has the prayer been shortened?" He said, "What is that?" He said, "You prayed three rak’ahs." So he prayed a rak’ah, then he performed the two prostrations of forgetfulness, then he performed the taslim. And he mentioned that Abu Hanifah said: If he speaks, he does not perform them (the two prostrations).