Shaqiq narrates: It was said to Sayyiduna Ali (may Allah be pleased with him): "Why do you not appoint someone as your successor?" He said: "Did the Prophet (peace and blessings be upon him) appoint anyone as his successor, that I should appoint a successor over you? If Allah Almighty intends good for the people, He will gather them around the best of them, just as Allah Almighty gathered the people, after their Prophet, around the best of them."
Bazzar recorded it, and its narrators are the narrators of the Sahih, except Isma'il bin Abi al-Harith, and he is thiqah (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14334