Muwatta Malik Book 8, Hadith 22
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."
Yahya related to me from Malik from Ibn Shihab that Abdullah ibn Amr ibn al As said, "When we arrived at Madina we were struck down by a severe epidemic which debilitated us greatly. The Messenger of Allah, may Allah bless him and grant him peace, came out to the people while they were praying nawafil prayers sitting down. The Messenger of Allah, may Allah bless him and grant him peace, said, 'The prayer of the one sitting is equal to only half the prayer of the one standing.' "
Yahya related to me from Malik from Hisham ibn Urwa from his father that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, told him that she had never seen the Messenger of Allah, may Allah bless him and grant him peace, doing night prayers sitting down until he was getting on in years. He would recite sitting down until when he wanted to go into ruku, he would stand up and recite about thirty or forty ayats and then go into ruku.