by Imam Malik, translation by A'isha `Abdarahman at-Tarjumana and Ya`qub Johnson , Volume: Prayer in Congregation
Chapter: The Excellence of the Prayer of Someone Standing over the Prayer of Someone Sitting
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 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.' "