She said, "I never saw the Messenger of Allah ﷺ perform his voluntary prayer sitting down until a year or two before his death. Then he began praying his voluntary prayer while seated, reciting a surah so slowly and deliberately that it would become longer than one originally longer than it."
حدثنا عبد الأعلى عن معمر عن الزهري عن السائب بن يزيد عن المطلب بن أبي وداعة
أنها قالت لم أر رسول الله ﷺ يصلي في سبحته جالسا قط حتى كان قبل موته بعام أو بعامين فكان يصلي في سبحته جالسا ويقرأ السورة فيرتلها حتى تكون أطول من أطول منها
Its isnad is sahih. It includes three companions, Saaib, Muttalib, and Hafsa. The hadith was narrated by Muslim, 1/507, no. 773, in the chapter on travelers, and Tirmidhi, 2/211, no. 373, in the chapter on prayer times, where he said, hasan sahih, and Nasai, 3/223, no. 1580, and Darimi, 1/373, no. 1385, in the chapter on prayer, and Ibn Khuzayma, 2/238, no. 1242.