Musnad Ahmad 1156:126

Yunus narrated to us, he said: Aban narrated to us, from Yahya ibn Abi Kathir, from Abu Salamah ibn Abd al-Rahman, from Umm Salamah, the wife of the Prophet, may God bless him and grant him peace, who said: "The Prophet, may God bless him and grant him peace, used to pray two rak'ahs after the Dhuhr prayer. A delegation came to him and occupied him, so he did not pray them. Then he prayed them after the Asr prayer."