Shaqiq bin Salamah said that it was said to Sayyiduna Ali (may Allah be pleased with him): Appoint a successor over us. He said: The Prophet (peace and blessings be upon him) did not appoint a successor, so how should I appoint one? But if Allah intends good for His servants, He will unite them upon what is good, just as He united them after their Prophet.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16573