It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that she said: "Sayyiduna Ali (may Allah be pleased with him) was upon the truth; whoever followed him followed the truth, and whoever abandoned him abandoned the truth. This was a covenant already fixed before this day of his."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20062