Sahih Bukhari Volume 1, Book 9, Hadith 590

حَدَّثَنَا أَبُو نُعَيْمٍ، قَالَ حَدَّثَنَا عَبْدُ الْوَاحِدِ بْنُ أَيْمَنَ، قَالَ حَدَّثَنِي أَبِي أَنَّهُ، سَمِعَ عَائِشَةَ، قَالَتْ وَالَّذِي ذَهَبَ بِهِ مَا تَرَكَهُمَا حَتَّى لَقِيَ اللَّهَ، وَمَا لَقِيَ اللَّهَ تَعَالَى حَتَّى ثَقُلَ عَنِ الصَّلاَةِ، وَكَانَ يُصَلِّي كَثِيرًا مِنْ صَلاَتِهِ قَاعِدًا ـ تَعْنِي الرَّكْعَتَيْنِ بَعْدَ الْعَصْرِ ـ وَكَانَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّيهِمَا، وَلاَ يُصَلِّيهِمَا فِي الْمَسْجِدِ مَخَافَةَ أَنْ يُثَقِّلَ عَلَى أُمَّتِهِ، وَكَانَ يُحِبُّ مَا يُخَفَّفُ عَنْهُمْ‏

Narrated By 'Aisha : By Allah, Who took away the Prophet. The Prophet never missed them (two Rakat) after the 'Asr prayer till he met Allah and he did not meet Allah till it became heavy for him to pray while standing so he used to offer most of the prayers while sitting. (She meant the two Rakat after Asr) He used to pray them in the house and never prayed them in the mosque lest it might be hard for his followers and he loved what was easy for them.