Musnad Ahmad 1153:1833

Aswad bin Amir narrated to us, from Sharik, from Ibrahim, from Mujahid, from the freed slave woman of As-Sa’ib, from Aisha, and Layth, from Mujahid, from the freed slave woman of As-Sa’ib, from Aisha, who attributed it [to the Prophet], she said: He said: "The prayer of the one sitting is half of the prayer of the one standing, [provided it is] not cross-legged."