Musnad Ahmad 28:179

Hassan narrated to us, Ibn Lahi'ah narrated to us, Bakr ibn 'Amr narrated to us, from Abu 'Abd al-Rahman al-Hubuli, from 'Abdullah ibn 'Amr, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Hearts are vessels, and some of them are more retentive than others. So when you ask Allah, the Almighty and Majestic, O people, ask Him while you are certain of the response. For indeed, Allah does not respond to a servant who calls upon Him from the back of a heedless heart."