Musnad Ahmad 1155:20

'Abd al-Rahman ibn Mahdi narrated to us, Malik ibn Anas narrated to us, from al-Zuhri; and 'Abd al-Razzaq informed us, Ma'mar informed us, from al-Zuhri, from al-Sa'ib ibn Yazid, from al-Muttalib ibn Abi Wada'ah, from Hafsah, who said: I never saw the Messenger of Allah, peace and blessings of Allah be upon him, praying his voluntary prayer sitting down at all, until it was a year before his death. He would pray sitting down, and he would recite the surah and chant it (tartil), until it would be longer than one longer than it.