Musnad Ahmad 30:747

Muhammad ibn Bishr narrated to us, Hisham ibn Urwah narrated to us, Salih ibn Abi Salih as-Samman narrated to us, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No one bears the hardship of Medina and its difficulty, except that I will be an intercessor for him and a witness, or a witness and an intercessor."