Musnad Ahmad 30:1568

Ishaq narrated to us, Abu 'Awanah narrated to me, from 'Umar ibn Abi Salamah, from his father, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you wishes, let him look at what he is wishing for, for he does not know what will be written for him from his wish."