Musannaf Ibn Abi Shayba 3:2257

Ibn Idris narrated to us, from Ubayd Allah ibn Umar, from al-Zuhri, from Ibn Umar, who said: We arrived in Medina and an epidemic afflicted us, until we prayed while sitting. The Prophet (peace and blessings of Allah be upon him) said: "The prayer of the one sitting is half of the prayer of the one standing."