Musnad Abu Ya'la al-Mawsili 210:24

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Malik, from al-Zuhri, from al-Sa’ib ibn Yazid, from al-Muttalib ibn Abi Wada’ah, from Hafsah, who said: I did not see the Messenger of Allah, may Allah bless him and grant him peace, pray a voluntary prayer sitting down until 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.