Sunan an-Nasa'i, Volume 1, Book 6, Hadith 579 (DarusSalam) ⓘ

It was narrated from Abu Salamah that he asked 'Aishah about the two prostrations (Rak'ahs) that the Messenger of Allah used to pray after 'Asr. She said: "He used to pray them before 'Asr, but if he got distracted or forgot them, he would pray them after 'Asr, and if he did a prayer he would be constant in it."

أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ أَبِي حَرْمَلَةَ، عَنْ أَبِي سَلَمَةَ، أَنَّهُ سَأَلَ عَائِشَةَ عَنِ السَّجْدَتَيْنِ اللَّتَيْنِ، كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّيهِمَا بَعْدَ الْعَصْرِ فَقَالَتْ إِنَّهُ كَانَ يُصَلِّيهِمَا قَبْلَ الْعَصْرِ ثُمَّ إِنَّهُ شُغِلَ عَنْهُمَا أَوْ نَسِيَهُمَا فَصَلاَّهُمَا بَعْدَ الْعَصْرِ وَكَانَ إِذَا صَلَّى صَلاَةً أَثْبَتَهَا ‏.‏