Musnad Ahmad 1153:1885

Abu Kamil narrated to us, Zuhayr narrated to us, Ibrahim ibn al-Muhajir al-Bajali narrated to us, from Mujahid, that al-Sa’ib asked Aisha and said: "I am not able to pray except while sitting, so what is your opinion?" She said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'The prayer of a man while sitting is like half of his prayer while standing.'"