Musnad Ahmad 32:455

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