Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) said: "These hearts are vessels, and the best of them is the one that preserves the most. When you ask of Allah Almighty, ask Him in a state in which you have firm confidence (that is, certainty) that the supplication will be answered, for Allah Almighty does not answer the supplication of a servant who supplicates with a heedless heart."
Imam al-Tabarani recorded this narration; its chain contains a narrator, Bashir bin Maymun al-Wasiti, and there is agreement on his being da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدعية / 17205