Muwatta Imam Malik Book of Prayer in Congregation, Hadith 22

Yahya related to me from Malik from Ibn Shihab from as-Sa'ib ibn Yazid from al Muttalib ibn Abi Wadaa as-Sahmi that Hafsa, the wife of the Prophet ﷺ said, "I never saw the Messenger of Allah ﷺ praying nawafil sitting, until a year before his death, when he began to pray them sitting. He would recite the sura with a measured slowness so that it would seem to be longer than other suras which were actually longer than it."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنِ السَّائِبِ بْنِ يَزِيدَ، عَنِ الْمُطَّلِبِ بْنِ أَبِي وَدَاعَةَ السَّهْمِيِّ، عَنْ حَفْصَةَ، زَوْجِ النَّبِيِّ ﷺ أَنَّهَا قَالَتْ مَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ صَلَّى فِي سُبْحَتِهِ قَاعِدًا قَطُّ حَتَّى كَانَ قَبْلَ وَفَاتِهِ بِعَامٍ فَكَانَ يُصَلِّي فِي سُبْحَتِهِ قَاعِدًا وَيَقْرَأُ بِالسُّورَةِ فَيُرَتِّلُهَا حَتَّى تَكُونَ أَطْوَلَ مِنْ أَطْوَلَ مِنْهَا .