At this unexpected declaration, all the people present stared at each other in stupefaction; and knowing not what to reply, their features betrayed naught else but fierce enmity and showed what their answer would be. Hazrat Ali, at length rose up and declared: "O Prophet of Allah! I will share thy work and help thee in thy mission."
Prophet Muhammad embraced Hazrat Ali with great affection and proclaimed: "Here is my proxy and my lieutenant. Here standeth by brother. Listen to him and obey". At this occasion, known as 'Dawath-e-Asheera' (propagation of the religion amongst the relatives), Hazrat Ali was declared by the Holy Prophet as the Proxy and Deputy. The value of Hazrat Ali's support to the Holy Prophet and his designation as the Vicegerent at this stage is fairly well assessed by theologians, historians and thinkers of the West and the East. Reference:
(1) Taiseer-e-Tabari Vol. 19 page 68
(2) Tatseer-e-Mo'allam-ul-Thunzeel page 663
(3) Masnad Imam Ahmed Ibne Humbal Vol. I page 163
(4) Musthadrik Imam Hakim Vol. III page 133
(5) History of Tabari Vol. II page 216
(6) History of Kamil Vol. II page 26
(7) History of Abdul Fida Vol. I page 116

(ii) The second occasion was at the time of the conquest of Khaibar. The Battle of Khaibar took place in Muhorrum of the year 7 A.H. The Muslim army succeeded in defeating the Jews in minor skirmishes but when they tried to capture the main fort of al-Kamus they had to face a set back; they could not succeed though they tried for days at stretch. The defeats sustained there were sadly demoralizing the Muslim Army. Holy Prophet Muhammad had allowed every important person to command the Muslim forces day by day, but each day the result was fresh defeat, fresh demoralization, fresh boldness of Jews and daily increasing danger of some more clans joining hands with the Jews. The Prophet sadly felt the need of Hazrat Ali at his side and when the news of the last disastrous setbacks of the Muslim army were brought to him, the Holy Prophet said; "Tomorrow, I shall give the command of this army to a man who is brave, who will keep on attacking, who will not run away from the battle-field, who loves God and his Prophet and is beloved by God and His Prophet and who will not come back to me without success."