Muwatta Malik Book 8, Hadith 21
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 Ismail ibn Muhammad ibn Said ibn Abi Waqqas from a mawla of Amr ibn al-As or of Abdullah ibn Amr ibn al-As that the Messenger of Allah, may Allah bless him and grant him peace, said, "The prayer of one of you sitting down is only equal to half the prayer of one of you when he is standing."
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."