Next day, all the companions gathered close to the Prophet with great expectations; but without glancing at the group, the Prophet sent for Hazrat Ali and handed over the command. Hazrat Ali took the fort by storm; killed Marhab, Anther, Murra, Harris and four other chieftains of the Jews in hand to hand combats. He then led his army inside the fort and within four hours flew the flag of the Holy Prophet on the biggest Jewish stronghold. The news of success so pleased the Holy Prophet that he, though ill, came out to greet the victor, embraced him and said, "O Ali you are from me and I am from you; you will inherit me and I will inherit you; you are unto me what Harun was unto Musa; enmity against you is enmity against me, war against you is war against me; your flesh is my flesh; your blood is my blood. As per orders of God I will give you these tidings that your friends will be rewarded in the Heaven and your enemies will be punished in the Hell". Can there by anything more forceful, more eloquent, categorical and more pregnant with clear indications than the words which the Holy Prophet used? Has the Holy Prophet ever used such words for anybody else?Reference:
(1) Moarej-un-Naboovat Vo. IV page 216
(2) Almanquible of Akhtab-e-Kharzami
(3) Ibne Hasham's Seerat
(4) Tabaris History
(5) Unabee-ul-Maoduth of Mulah Ali Hamdani.
(iii) The third occasion when Prophet Muhammad indicated Hazrat Ali as the Imam after him was when Sura Barat was to be read before the infidels of Mecca. This Sura declared that God and His Apostle would have nothing to do with the infidels and polytheists in the future. All the treaties which existed till then were annulled and cancelled. No polytheists or infidel would be allowed to enter the city of Mecca or the precincts of Ka'bah. Prophet Muhammad first ordered Hazrat Abu Bakr to carry this Sura to be read to the Meccans at Ka'bah, but soon afterwards he recalled him and gave the Sura to Hazrat Ali. When Abu Bakr complained about this change, the Apostle of God replied: "I  have done it under the order of God which said explicitly that either I should perform this duty or somebody who is like me."
Reference:
(1) Musthadrik Vo. III page 32 of Imam Hakim
(2) Riaz-ul-Nazarah Vo. II page 203
(3) Masnad-e-Imam Ahmed Ibne Humbal Vol. I page 331
(4) Asaba-Fee-Ma'arafath-e-Sahaba Vo. IV, page 270
(5) Izaiath-ul-Khifa, Section 2, page 261