I heard ‘Ali in Ar-Rahbah, when he was adjuring the people and asking who had been present with the Messenger of Allah (ﷺ) on the day of Ghadeer Khumm. when he said what he said. Thirteen men stood up and testified that they had heard the Messenger of Allah (ﷺ) say; “If I am a person"s mawla”, ‘Ali is also his mawla."

(The word mawla refers to the one who manumits a slave, who has the right to inherit from the ex-slave. Changing one"s mawla means giving the right of inheritance to the new mawla)