Musnad Ahmad 1155:19

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from al-Sa'ib ibn Yazid, from al-Muttalib ibn Abi Wada'ah, from Hafsah, the wife of the Prophet, may Allah bless him and grant him peace, that she said: I never saw the Messenger of Allah, may Allah bless him and grant him peace, praying his voluntary prayer sitting down at all, until it was a year or two before his death. Then he would pray his voluntary prayer sitting down, and he would recite the surah and recite it slowly (tartil), until it would be longer than one longer than it.