Musnad Abu Ya'la al-Mawsili 209:68

Abu Musa Muhammad ibn al-Muthanna narrated to us, Yahya ibn Kathir narrated to us, Shu'bah narrated to us, from 'Abdullah ibn Abi al-Mujalid, from 'Abdullah ibn Shaddad ibn al-Had, from Umm Salamah, that she said: The Messenger of Allah (may Allah bless him and grant him peace) prayed two rak'ahs in my house. I said to him, "What are these two?" He said, "I used to pray them before the 'Asr [prayer]."