Musnad Ahmad 30:2838

Waki’ narrated to us, from Sufyan, from Abu al-Zinad, from al-A’raj, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not one of you say: 'O Allah, forgive me if You will; O Allah, have mercy on me if You will.' Rather, he should be firm in his request, for there is no one who can compel Him."