Musnad Ahmad 1153:1342

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: 'Abd Allah ibn 'Ubayd Allah said, and Ibn Bakr said: 'Ubayd Allah ibn Abi Mulaykah said: I heard the family of 'Aishah mentioning on her authority that she used to say: "The Messenger of Allah (peace and blessings of Allah be upon him) used to be very strenuous upon his body in worship. However, when he advanced in age and became heavy with flesh, the majority of his prayer was while he was sitting."