Musnad Ahmad 30:196

Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, who attributed it to the Prophet (may Allah bless him and grant him peace): "When one of you supplicates, let him not say: 'O Allah, forgive me if You will.' Rather, let him be firm in his request, for there is no one who can compel Him."