Musnad Abu Ya'la al-Mawsili 150:312

Yahya ibn Ayyub narrated to us, Isma'il ibn Ja'far narrated to us, Muhammad informed me, Abu Salamah informed me that he asked 'Aishah about the two prostrations (rak'ahs) that the Messenger of Allah (may Allah bless him and grant him peace) used to pray after the 'Asr prayer. She said: "He used to pray them, then he became occupied from them or forgot them, so he prayed them after 'Asr, and then he established them (made them a regular practice). And whenever he performed a prayer, he would establish it." Abu Zakariyya said: Isma'il said: "Meaning, he persisted in them."