When the Messenger of Allah (ﷺ) grew bulky and heavy he would observe (most of his Nafl) prayers sitting.
Sahih Muslim Book 6, Hadith 141
← Previous Hadith 'A'isha reported that the Messenger of Allah (ﷺ) died (in this very state) that he observed most of his (Nafl) prayers in a sitting position.

Sahih Muslim Book 6, Hadith 140 Next Hadith → Never did I see the Messenger of Allah (ﷺ) observing supererogatory prayer sitting till one year before his death when he would observe Nafl prayer in a sitting position, and he would recite the Surah (of the Qur'an) in such a slow-measured tone (that duration of its recital) became more lengthy than the one longer than this.

Sahih Muslim Book 6, Hadith 142