Musnad Ahmad 1153:2112

'Abd al-Samad narrated to us, he said: 'Umar ibn Abi Za'idah narrated to us, from Abu Ishaq, from al-Aswad, from 'A'ishah, she said: "The most of the Prophet's, peace and blessings of Allah be upon him, prayer was while sitting, except for the prescribed prayer. And the most beloved of deeds to him was that which a person persists in, even if it were little."