Musnad Ahmad 641
It was narrated that Zadhan Abu ‘Umar said:
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)
حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا عَبْدُ الْمَلِكِ، عَنْ أَبِي عَبْدِ الرَّحِيمِ الْكِنْدِيِّ، عَنْ زَاذَانَ أَبِي عُمَرَ، قَالَ سَمِعْتُ عَلِيًّا، فِي الرَّحْبَةِ وَهُوَ يَنْشُدُ النَّاسَ مَنْ شَهِدَ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَوْمَ غَدِيرِ خُمٍّ وَهُوَ يَقُولُ مَا قَالَ فَقَامَ ثَلَاثَةَ عَشَرَ رَجُلًا فَشَهِدُوا أَنَّهُمْ سَمِعُوا رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَهُوَ يَقُولُ مَنْ كُنْتُ مَوْلَاهُ فَعَلِيٌّ مَوْلَاهُ‏.‏