Abd al-Rahman bin Abi Layla narrates: I was present with Sayyiduna Ali (may Allah be pleased with him) when he was at al-Rahbah (this is the name of a place, or perhaps it means an open ground). He asked the people, adjuring them by Allah: I ask, by Allah, the person who heard the Prophet (peace and blessings be upon him) say on the day of Ghadir Khumm: "Whoever's mawla I am, Ali too is his mawla," to stand up and testify. The narrator Abd al-Rahman says: Twelve Companions who had fought at Badr stood up. The sight of one of them is still before my eyes; he was wearing trousers. They said: We testify that we heard the Prophet (peace and blessings be upon him) say on the day of Ghadir Khumm: Am I not closer to the believers than their own selves, and are my wives not their mothers? We said: Yes, O Messenger of Allah. Then the Prophet (peace and blessings be upon him) said: "Whoever's mawla I am, Ali too is his mawla. O Allah, love whoever loves him and be hostile to whoever is hostile to him."
Abu Ya'la recorded it, and its narrators have been declared trustworthy; Abdullah bin Ahmad also recorded it.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14615