Prophet Muhammad (s) appoints Ali (as) early in mission

Narrated Ibn Humayed, from Salamah, from Muhammad Ibn Is'haq, from Abd al-Ghaffar Ibn al-Qasim, from al-Minhal Ibn Amr, from abdallah Ibn al-Harith Ibn Nawfal Ibn al-Harith Ibn Abd al-Muttalib, from Abdallah Ibn Abbas, from Ali Ibn Abi Talib:

When the verse "And warn your close tribe (Quran 26:214)" was revealed to the prophet, he called me and said to me, "Ali, God has commanded me to warn my tribe of near kindred. I was troubled by this, since I knew that when I discuss the matter to them they would respond in a way which I would not like. I kept silent until Gabriel came to me and said "If you do not do what you are commanded, your Lord will punish you." So prepare a measure of wheat for us, add a leg of lamb to it, fill a large bowl of milk for us, and then invite sons of Abd al-Muttalib for me so that I may speak to them what I have been commanded to tell them."

I did what he had told me to do. At that time they numbered forty men more or less, including his uncles Abu Talib, Hamzah, al-Abbas, and Abu Lahab. When they had gathered together, he called me to bring the food which I had prepared. I brought it, and when I put it down, prophet took a piece of meat, broke it with his teeth, put it in the dish. Then he said, "Take in the name of God." They ate until they could eat no more, and yet the food was as it had been. I swear by God, in whose hand Ali's soul rests, that a single man could have eaten the amount of food which I prepared for them. Then he said "Give them something to drink." So I brought them the bowl and they drank from it until they became full, and I swear by God that one man could have drunk that amount. When prophet wanted to speak to them, Abu Lahab interrupted him and said "Your host has long since bewitched you." Then they dispersed without the prophet speaking to them.

On the following day he said to me "Ali, this man interrupted what I wanted to say so that people dispersed before I could speak to them. Prepare the same food for us as you did yesterday, and invite them here." I did this, and brought them food when he called me. He did as he had done the other day, and they ate until they could eat no more. Then he said, "Bring the bowl," and they drank until they could drink no more.

Then he spoke to them, saying, "Banu Abd al-Muttalib, I don't know of any young man among Arabs who has brought for his people something better than what I have brought to you. I bring the best of this world and the world after, since God has commanded me to summon you to him. Which of you will aid me in this matter, so that he will be my brother, my executor (Wasi), my successor (Caliph) among you?" They all held back, and even though I was the youngest, I said "I will be your helper, O' prophet of God." He put his hand on the back of my neck and said "This is my brother, my executor (Wasi), my successor (Caliph) among you, so listen to him and obey him." They rose up and saying to Abu Talib, "He has commanded you to obey your son and to obey him!"

Sunni References:
(1) History of al-Tabari, English version, v6, pp 88-91
(2) History of Ibn Athir, v2, p62
(3) History of Ibn Asakir, v1, p85
(4) Tafsir al-Durr al-Manthoor, by al-Hafidh Jalaluddin al-Suyuti, v5, p97
(5) Tafsir al-Khazin, by Ala'uddin al-Shafi'i, v3, p371
(6) Shawahid al-Tanzil, by al-Hasakani, v1, p371
(7) Kanz al-Ummal, by al-Muttaqi al-Hindi, v15, p15, pp 100-117
( al-Sirah al-Halabiyah, v1, p311
(9) Dala'il al-Nabawiyyah, by al-Baihaqi, v1, pp 428-430
(10) al-Mukhtasar, by Abul Fida, v1, pp 116-117
(11) Life of Muhammad, by Hasan Haykal, p104 (First Arabic Edition only. In the second edition the last sentence of Prophet is deleted.)
(12) Tahdhib al-Athar, v4, pp 62-63.