Sayyidah Umm Salamah (may Allah be pleased with her) said: "Ali is upon the truth; whoever follows him has followed the truth, and whoever abandons him has abandoned the truth. This is a covenant that was made before (the Prophet's (peace and blessings be upon him)) death."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20248