Musnad Abu Ya'la al-Mawsili 118:48

Ahmad ibn al-Miqdam al-'Ijli narrated to us, Muhammad ibn Bakr narrated to us, Ibn Jurayj narrated to us, from Ibn Shihab, from Anas ibn Malik, who said: The Prophet, peace and blessings of Allah be upon him, arrived in Medina while it was feverish, and the people were afflicted with fever. The Prophet, peace and blessings of Allah be upon him, entered the mosque while the people were sitting and praying. He said: "The prayer of the one sitting is half of the prayer of the one standing." He said: So the people exerted themselves to pray while standing.