Unaysah bint Zayd bin Arqam narrates from her father, Sayyiduna Zayd bin Arqam (may Allah be pleased with him), who said: the Messenger of Allah (peace and blessings be upon him) gave orders concerning the trees, so the ground beneath them was swept clean and sprinkled with water. Then he (peace and blessings be upon him) delivered a sermon to us. By Allah! There was nothing that will happen until the Hour is established except that he (peace and blessings be upon him) informed us of it that day. Then he said: "O people! Who has more right over you than your own selves?" We said: "Allah and His Messenger have more right over us than our own selves." He said: "Then whoever I am his mawla, this one (Ali) is his mawla" - meaning Sayyiduna Ali (may Allah be pleased with him). Then he (peace and blessings be upon him) took hold of his hand and raised it, then said: "O Allah, befriend whoever befriends him and be hostile to whoever is hostile to him."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5109