Musnad Abu Ya'la al-Mawsili 209:141

Abu Khaythama narrated to us, Bahlul ibn Muwarriq al-Shami narrated to us, Musa ibn Ubaydah narrated to us, who said: Thabit, the freed slave of Umm Salamah, informed me, from Umm Salamah, the wife of the Prophet (may Allah bless him and grant him peace): That the Messenger of Allah (may Allah bless him and grant him peace), when he would leave his house, would pray two rak'ahs before Dhuhr, and he would pray two rak'ahs before Asr. The Messenger of Allah (may Allah bless him and grant him peace) sent a collector to a people. When he reached them, a group of them wanted to assist him and prepared themselves for that. When the collector arrived and saw the people, he thought they were going to kill him, so he returned to the Messenger of Allah (may Allah bless him and grant him peace) and said: "They withheld their charity from me." The collector was detained from the people, so they came to the Messenger of Allah (may Allah bless him and grant him peace) to apologize to him. He had already performed the Dhuhr prayer, and they continued to apologize to him until he prayed Asr, and he forgot the two rak'ahs that he used to pray before Asr. Aisha sent word to Umm Salamah: "O sister, what are the two rak'ahs that the Messenger of Allah (may Allah bless him and grant him peace) prayed in your room after Asr?" She informed her and said: "I have not seen the Messenger of Allah (may Allah bless him and grant him peace) pray before them nor after them."