Maymun Abu Abdullah narrates: Sayyiduna Zayd bin Arqam (may Allah be pleased with him) said, while I was listening: We halted with the Messenger of Allah (peace and blessings be upon him) in a valley called the valley of Khumm. He (peace and blessings be upon him) ordered the prayer and offered it in the intense midday heat. Then he addressed us, and shade was made for the Messenger of Allah (peace and blessings be upon him) from the sun by spreading a cloth over a tree. He (peace and blessings be upon him) said: "Do you not know that I have a greater claim over every believing man and woman than they have over themselves?" The Companions said: "Certainly." He said: "Then whoever I am his mawla, Ali is his mawla. O Allah, befriend whoever befriends him, and be hostile to whoever is hostile to him."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5073