Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1659 (DarusSalam) ⓘ

It was narrated that Hafsah said: "I never saw the Messenger of Allah offer his voluntary prayers sitting down until one year before his death. Then he used to pray sitting down, reciting the surah so slowly that it seemed to be longer than a surah that is longer."

أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنِ السَّائِبِ بْنِ يَزِيدَ، عَنِ الْمُطَّلِبِ بْنِ أَبِي وَدَاعَةَ، عَنْ حَفْصَةَ، قَالَتْ مَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم صَلَّى فِي سُبْحَتِهِ قَاعِدًا قَطُّ حَتَّى كَانَ قَبْلَ وَفَاتِهِ بِعَامٍ فَكَانَ يُصَلِّي قَاعِدًا يَقْرَأُ بِالسُّورَةِ فَيُرَتِّلُهَا حَتَّى تَكُونَ أَطْوَلَ مِنْ أَطْوَلَ مِنْهَا ‏.‏