Musnad Ahmad 32:40

Isma'il narrated to us, 'Abd al-'Aziz narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you supplicates, let him be firm in his supplication, and let him not say: 'O Allah, if You will, then give me,' for there is no one who can compel Allah."