Musnad Ahmad 30:2849

Narrated to us Abd al-Rahman, from Sufyan, from Abu al-Zinad, from al-A’raj, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Let not one of you say: 'O Allah, forgive me if You will,' for Allah, the Almighty and Majestic, has no one to compel Him. But let him be firm in his request."