Prophet Muhammad was worried about people rebelling against him if he announced Imam Ali as his successor, so he delayed following the message to formalize his succesorship as long as he was allowed to.

The wisdom of this showed by how during the hajj. Some companions of the Prophet had suspected the Prophet would announce Imam Ali as his successor at hajj and whenever the prophet would give a sermon that veered towards this topic, they would become agitative and make noise in order to derail the announcement.

After the hajj, the Muslims started travelling back towars their home towns, with the Yemenis going south, and the Meccans remaining behind in Mecca, while the prophet and others went north to Medina. At this time Gabriel once again conveyed Allah's command to the Prophet to announce his successor and this time also promised His protection. Perphaps after the groups had split the munafiqeen were in too small a number to cause any trouble, and Prophet Muhammad announced Imam Ali as his successor.

Lecture notes available at http://www.why-quran.org/?p=766.

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In the last months of the prophet's life, Gabriel descended to the prophet conveying a message from Allah: "There remain two obligations that you must convey to your people: the obligation of Hajj, and the obligation of Wilayah (the guardianship and succession) after you. For indeed, I have never left My earth without a proof, and I will never leave it without one." The Prophet had often spoken about Imam Ali's unique rank, but hadn't formalized his succession in front of a large audience.

Prophet Muhammad ordred all who could to join him in his Hajj, about 70-124,000 people joined, and sent a message to Imam Ali, who was still in Yemen, to also join with his men.

When nearing Mecca, Imam Ali rushed ahead to meet the Prophet, leaving Abu Rafi al-Qibti in charge of the men. When Imam Ali returned he saw that for their Ihram those men had put on the fine Yemeni fabrics, a khums collected from Yemen. Imam Ali reprimanded them and ordered them removed and repacked immediately. While this was the just course of action, this disciplining would natrually lead to tension being created between him and his troops.

After the they all returned to Mecca, Imam Ali was praying in the sacred mosquen when a begger entered and asked for charity. Without interrupting his prayer, from ruku Imam Ali extendeded his hand and offered his ring to the beggar. At this moment some companions were with the prophet as the verse was revealed "Your guardian is only Allah, his Prophet, and the faithful who maintain the prayer and give charity while bowing down."

During hajj, the prophet often emphasized the two weightly things he was leaving behind: the Quran and the Ahlul Bayt. During this time Allah repeatedly instructed the prophet to make his succession announcement, but concerns about the potential backlash from some companions kept Prophet Muhammad from doing so until Allah sent a promise of protection.

The Prophet's concern about backlash from his companions foreshadowed the opposition Imam Ali would later encounter.

Lecture notes available at http://www.why-quran.org/?p=762.

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Prophet Muhammad sent a mission to Yemen to invite them to Islam. The first mission was led by Khalid ibn Al-Walid, who was a harsh man who had fought against the Muslims in the Battle of Uhud, but he had an aristrocratic background and by making him feel valued Prophet Muhammad had hoped to turn him into a productive member of the Muslim community.

However once in Yemen, instead of following the prophetic principles of dawah Khalid hastily resorted to violence. The Tribe of Numayr, feeling threatened, sent a delegation to the Prophet in Medina to ask for protection from Khalid.

The Prophet sent Imam Ali to Yemen to do damage control and instructed Khalid's men to follow him instead. While sending Imam Ali he gave him various bits of advice, emphasizing the need to display wisdom, to soften people's hearts, and to remain upright.

The prophet emphasized the importance of avoiding bloodshed if possible, to the point where he said to not even engage in battle unless they kill one of your men first, and to even then first give them a chance to repent.

Lecture notes available at http://www.why-quran.org/?p=754.

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