Maliks Muwatta Book 8, Hadith Number 21.
Section : The Excellence of the Prayer of Someone Standing over the Prayer of Someone Sitting.
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.'”