Musnad Ahmad 1153:2129

'Abd al-Malik ibn 'Amr narrated to us, from Zuhayr, from 'Abd Allah ibn Abi Bakr, from his father, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, used to go out to al-Baqi' and supplicate for them. 'Aishah asked him about that, and he said: "Indeed, I have been commanded to supplicate for them."