Musnad Abu Ya'la al-Mawsili 150:437

Ali ibn al-Ja'd narrated to us, Zuhayr informed us, from Ibrahim ibn al-Muhajir, from Mujahid, that al-Sa'ib asked Aisha and said: "I have grown old, and I am unable to pray except while sitting, so what is your opinion?" She said: "I heard the Messenger of Allah—or he said: The Messenger of Allah, peace and blessings of Allah be upon him—say: 'The prayer of the one sitting is half of his prayer while standing.'"