Sayyiduna Zayd bin Arqam (may Allah be pleased with him) narrates: Sayyiduna Ali (may Allah be pleased with him) adjured the people in al-Rahabah, asking that whoever had heard the Messenger of Allah (peace and blessings be upon him) say what he (peace and blessings be upon him) said about him should stand up. Sixteen men stood up and testified that they had heard the Messenger of Allah (peace and blessings be upon him) say: "O Allah! Whoever I am the mawla (patron) of, Ali is his mawla. O Allah! Befriend whoever befriends him, and be hostile to whoever is hostile to him." Sayyiduna Zayd bin Arqam (may Allah be pleased with him) says: I was among those who concealed (the testimony), and my eyesight was lost; Sayyiduna Ali (may Allah be pleased with him) had supplicated against whoever concealed it.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4966