Have a look at the revamped Hadith Database.
Bismillāh.
وَاجْعَل لِّي وَزِيرًا مِّنْ أَهْلِي هَارُونَ أَخِي اُشْدُدْ بِهِ أَزْرِي وَأَشْرِكْهُ فِي أَمْرِي
Appoint for me a minister from my family, Hārūn, my brother. Strengthen my back through him and make him my associate in my task.
Sūrat TāHā No. 20 Āyat 29-31
When Nabī Mūsā ‘alayhis-salām was told by Allah ‘azza wajall that he had been appointed a Prophet and had to communicate the message of monotheism to Fir‘awn, he asked Allah for a few things that he needed to do this task well. Among the requests he made was for a helper from his family. He asked that Hārūn, his brother, would be his wazir, to partake in his mission and be his support. According to ‘Allāmah Tabātabā’ī in Tafsīr al-Mīzān, the word wazir could mean one who carries the weight of a task or mission. It could also mean a place of refuge. Prophet Moses explained his request to Allah, that Hārūn would strengthen his back and share in his task.
In another verse of the Quran, the request of Prophet Mūsā (a) is explained thus: And my brother Aaron is more fluent than me in tongue, so send him with me as support, verifying me. Indeed, I fear that they will deny me (Q 28:34). Allah granted Mūsā his request and made Hārūn his wazir, i.e., deputy. The Quran says: You have been granted your request, O Moses. (Q 20:36). These verses establish the fact that the deputy of a Prophet is by Divine appointment. Prophet Mūsā did not pick his own deputy. He asked Allah subhānahu wata‘ālā and the Almighty God accepted his request. Only then did Hārūn become his wazir.
Prophet Muhammad sallallāhu ‘alayhi wa-ālihi wasallam also had a wazir. He told Imam Ali ‘alayhis-salām: O Ali, you are to me of the status of Aaron was to Moses, except that there is no prophet after me. This is the Hadith al-Manzilah, stating that Imam Ali (a) was the wazir of the Prophet (s). He would partake in the mission of the Prophet (s) and be his support. The Prophet (s) was Allah’s most beloved creature. The task given to him was heavier and more complex than that given to any other prophet. The fact that Ali (a) was appointed as the wazir of the Prophet (s) is a great honor for him, showing his ability to shoulder such a mission. He was the most qualified to take on that position.
Imam Ali (a) was the wazir of the Prophet (s) as well as his wasī (legatee). The event of Ghadir finalized the appointment of Imam Ali (a) as the successor of the Prophet (s). It took place on 18th of Dhul-hijjah, in the year 10 A.H. The Prophet (s) was returning from his final pilgrimage and asked the people to stop at the pond of Ghadīr. There he announced Ali (a) as the walī, uttering his famous words: Man Kuntu Mawlāhu Fa-Hadhā ‘Alīyun Mawlāhu– Of whomsoever I am his Master (mawlā), Alī is (also) his Master. It was a culmination of a series of announcements of the position of Imam Ali (a) as the helper of the Prophet (s) who would continue his mission after his death. This is blessed occasion of Eid al Ghadīr, is a day of happiness for all Muslims, especially those who follow the Ahlul Bayt ‘alayhimus-salām.
Source: Allāmah Muhammad Husayn Tabātabā’ī, Tafsīr al-Mīzān.