Jami` at-Tirmidhi 373
جامع الترمذي#373SahihHafsah, the wife of the Prophet (S), narrated:"I did not see Allah's Messenger (S) praying voluntary prayers sitting until the year before he died. Then he would perform Salat for the voluntary prayers sitting, and he would recite a Surah and prolong it such that it would be longer than the longest of them
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiSahih Muslim
In-book reference: 2:225