I never saw the Messenger of Allah ﷺ perform his voluntary prayers sitting until a year before his death. During that final year he would pray his voluntary prayers sitting, reciting a surah and drawing it out so slowly that it became longer than one originally longer than it.
حدثنا الأنصاري حدثنا معن حدثنا مالك بن أنس عن ابن شهاب عن السائب بن يزيد عن المطلب بن أبي وداعة السهمي
ما رأيت رسول الله ﷺ في سبحته قاعدا حتى كان قبل وفاته بعام فإنه كان يصلي في سبحته قاعدا ويقرأ بالسورة ويرتلها حتى تكون أطول من أطول منها
Muslim also narrated it in "Prayer of the Traveler," chapter on the permissibility of voluntary prayer standing or sitting, etc., no. 733, through the hadith of Malik, and it is found in Muwatta: 1/137. On this subject there is also a hadith from Umm Salama, perhaps referring to the hadith of Abdul-Razzaq, no. 4125, and from Anas bin Malik (mentioned previously: 371, on this subject).Hadith, "That he used to pray at night sitting..." etc., will come at 374, and "That he prays sitting..." etc., will come at 375.