Musnad Abu Ya'la al-Mawsili 148:86

Abu Bakr ibn Abi Shaybah narrated to us, 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 Messenger of Allah (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."