Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1816
Abu Salama asked 'A'isha about the two prostrations (i. e. rak'ahs) which the Messenger of Allah (may peace be upon him) 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.