Musnad Ahmad 1153:1871

'Abd al-Razzaq narrated to us, he said: al-Thawri informed us, from al-A'mash, from 'Umarah, from Yahya ibn al-Jazzar, from 'Aishah, she said: The Prophet, peace and blessings of Allah be upon him, used to pray nine rak'ahs at night. Then, when he became heavy and aged, he prayed seven.