Sahih Muslim Book - 6, Hadith - 1712

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ عَنِ السَّائِبِ بْنِ يَزِيدَ عَنِ الْمُطَّلِبِ بْنِ أَبِى وَدَاعَةَ السَّهْمِىِّ عَنْ حَفْصَةَ أَنَّهَا قَالَتْ مَا رَأَيْتُ رَسُولَ اللَّهِ -صلى الله عليه وسلم- صَلَّى فِى سُبْحَتِهِ قَاعِدًا حَتَّى كَانَ قَبْلَ وَفَاتِهِ بِعَامٍ فَكَانَ يُصَلِّى فِى سُبْحَتِهِ قَاعِدًا وَكَانَ يَقْرَأُ بِالسُّورَةِ فَيُرَتِّلُهَا حَتَّى تَكُونَ أَطْوَلَ مِنْ أَطْوَلَ مِنْهَا.

It was narrated that Hafsah said: "I did not see the Messenger of Allah (s.a.w) offer any voluntary prayer while sitting until one year before his death, when he would offer his voluntary prayers sitting, and he would recite a Surah at such a slow, measured pace that it would seem longer than another one which was longer."