Muwatta Malik Book 8, Hadith 20
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 Hisham ibn Urwa from his father that the Messenger of Allah, may Allah bless him and grant him peace, came out during his illness, and approached and found Abu Bakr standing, leading the people in prayer. Abu Bakr began to go back, but the Messenger of Allah, may Allah bless him and grant him peace, indicated to him to stay where he was. The Messenger of Allah, may Allah bless him and grant him peace, sat by the side of Abu Bakr, and Abu Bakr prayed following the prayer of the Messenger of Allah, may Allah bless him and grant him peace, who was sitting, and the people prayed following the prayer of Abu Bakr.
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.' "