Musnad Abu Ya'la al-Mawsili 155:103

On the authority of Abu Hurayrah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No one bears its hardship and its difficulty except that I will be an intercessor for him or a witness on the Day of Resurrection. And no one leaves it, desiring something else, except that Allah will replace it for him with that which is better than it. And Medina is better for them, if only they knew."