Yahya related to me from Malik from Ibn Shihab that Abdullah ibnAmr ibn al As said, "When we arrived at Madina we were struck down bya severe epidemic which debilitated us greatly. The Messenger ofAllah, may Allah bless him and grant him peace, came out to the peoplewhile they were praying nawafil prayers sitting down. The Messenger ofAllah, may Allah bless him and grant him peace, said, 'The prayer ofthe one sitting is equal to only half the prayer of the one standing.'"
App reference: Book 8, Hadith 21
USC-MSA web (English) reference: Book 8, Hadith 21
Arabic reference: Book 8, Hadith 310