Musnad Ahmad 30:3180

Ishaq narrated to us, he said: Malik informed us, and I read to Abd al-Rahman: Malik, from Abu al-Zinad, from al-A’raj, from Abu Hurairah, that 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.' Let him be firm in his request." Both of them said: "For there is no one to compel Him."