It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "The greatest name of Allah, by which, when He is supplicated, He answers, is in this verse of Surah Al Imran: 'Say: O Allah, Owner of sovereignty, You give sovereignty to whom You will' [Surah Al Imran: 26], to the end (of the verse)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12737