Maliks Muwatta Book 8, Hadith Number 22.

Section : Praying Voluntary Prayers (Nawafil) Sitting.

Yahya related to me from Malik from Ibn Shihab from as-Sa’ib ibn Yazid from al Muttalib ibn Abi Wadaa as-Sahmi that Hafsa, the wife of the Prophet, may Allah bless him and grant him peace, said, “I never saw the Messenger of Allah, may Allah bless him and grant him peace, praying nawafil sitting, until a year before his death, when he began to pray them sitting. He would recite the sura with a measured slowness so that it would seem to be longer than other suras which were actually longer than it.”