Yahya bin Ja'dah narrates from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: We set out with the Messenger of Allah (peace and blessings be upon him) until we reached Ghadir Khumm. He (peace and blessings be upon him) ordered that (the ground under the trees) be cleared; no day had ever come upon us hotter than that day. He (peace and blessings be upon him) praised and extolled Allah, then said: "O people! No prophet was ever sent except that he lived half the lifespan of the prophet before him, and it is near that I will be called and I will respond. I am leaving among you something after which you will never go astray: the Book of Allah." Then he (peace and blessings be upon him) stood up, took Ali (may Allah be pleased with him) by the hand and said: "O people! Who has more right over you than your own selves?" The Companions said: "Allah and His Messenger know best." He said: "Whoever I am the mawla (patron) of, Ali is his mawla."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4967