Musannaf Ibn Abi Shayba 3:2262

Mu'alla ibn Mansur and Khalid ibn Makhlad narrated to us, from Abdullah ibn Ja'far, from Isma'il ibn Muhammad ibn Sa'd, from Anas ibn Malik, who said: The Prophet, may Allah bless him and grant him peace, said: "The prayer of the one sitting is like half of the prayer of the one standing."