From Sahih Muslim (Hadith 1934):
Abu Salama asked 'A'isha about the two prostrations (i. e. rak'ahs) which the Messenger of Allah (ﷺ) made after the 'Asr. She said:He (the Holy Prophet) observed them before the 'Asr prayer, but then he was hindered to do so, or he forgot them and then he observed them after the 'Asr, and then he continued observing them. (It was his habit) that when he (the Holy Prophet) observed prayer, he then continued observing it. Isma'il said: It implies that he always did that
Reference: Muslim 1934