Sahih Muslim 835 a
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.
حَدَّثَنَا يَحْيَى بْنُ أَيُّوبَ، وَقُتَيْبَةُ، وَعَلِيُّ بْنُ حُجْرٍ، قَالَ ابْنُ أَيُّوبَ حَدَّثَنَا إِسْمَاعِيلُ، - وَهُوَ ابْنُ جَعْفَرٍ - أَخْبَرَنِي مُحَمَّدٌ، - وَهُوَ ابْنُ أَبِي حَرْمَلَةَ - قَالَ أَخْبَرَنِي أَبُو سَلَمَةَ، أَنَّهُ سَأَلَ عَائِشَةَ عَنِ السَّجْدَتَيْنِ اللَّتَيْنِ، كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّيهِمَا بَعْدَ الْعَصْرِ فَقَالَتْ كَانَ يُصَلِّيهِمَا قَبْلَ الْعَصْرِ ثُمَّ إِنَّهُ شُغِلَ عَنْهُمَا أَوْ نَسِيَهُمَا فَصَلاَّهُمَا بَعْدَ الْعَصْرِ ثُمَّ أَثْبَتَهُمَا وَكَانَ إِذَا صَلَّى صَلاَةً أَثْبَتَهَا ‏.‏ قَالَ يَحْيَى بْنُ أَيُّوبَ قَالَ إِسْمَاعِيلُ تَعْنِي دَاوَمَ عَلَيْهَا ‏.‏