Harun accepts that Imam Musa (as) is the true Imam

al-Ma’mun related that to them, saying: “One year I went with him (al-Rashid) to perform the hajj. When he arrived in Medina, he said: ‘Let no man from its inhabitants or from the Meccans whether they are of the children of the Emigrants and the Ansar or of the Banu Hashim unless he makes me know his lineage and his family.’ The delegates came to him one by one. They made the chamberlain know their lineage. Harun gave them permission (to meet him) and gave them gifts according to their rank and position. One day al-Fedl b. al-Rabi‘, his chamberlain, walked towards him and said to him: ‘There is a man at the door; he claims that he is Musa b. Ja‘far b. Muhammed b. ‘Ali b. al-Husayn b. ‘Ali b. Abi Talib.’
“When Harun heard of that, he ordered those sitting in his gathering to cling to respect and calmness. Then he said to his chamberlain: ‘Permit him; and let him not dismount except on my carpet.’”
The Imam, peace be on him, walked towards (Harun). Al-Ma’mun described him, saying: “Surely he was an old man; worship exhausted him. He looked like an old, small (water) skin; prostration injured his face. When Harun saw him, he rose for him. The Imam wanted to dismount, but al-Rashid shouted: ‘No by Allah, (you should not dismount) except on my carpet.’ So the chamberlain prevented him from dismounting. We looked at him with admiration and magnification. He walked riding his mount to the carpet. The chamberlains and the leading figures of the people surrounded him. Harun received him. He kissed him on the face and the eye. He took him by the hand and seated him in the front of his gathering. He asked him about his conditions, talked with him, and said to him:”
-O Abu al-Hasan, how may (persons) are dependent on you?
-Over five hundreds.
-Are they your children?
-Most of them are followers and servants. As for my children, they are
thirty and some.
-Why do you not marry the women to their cousins?
-The hand falls short of that (lit. I do not have money).
-What about the country estate?
-It sometimes produces and sometimes does not produce.
-Are you indebted?
-Yes.
-How much?
-About ten thousand dinars.
-Cousin, I will give you an amount of money to marry your children and to
plant your country estates.
-You have linked your womb (relatives). Allah has thanked you for this beautiful intention. The blood relationship is touching and close. The lineage is one. Al-‘Abbas is the uncle of the Prophet, may Allah bless him and his family, and full brother of his father. He is the uncle of ‘Ali b. Abi Talib and full brother of his father. May Allah not send you far from doing that. He has opened you hand, made your origin honorable, and made high your lineage.
-Certainly, I will do that, Abu al-Hasan.
-Surely Allah, the Great and Almighty, has made it incumbent on rulers to relive the poor from among the community, to pay the debts of the indebted, and to clothe those naked. You are appropriate to do that.
-I will do that, Abu al-Hasan.
Then the Imam, peace be on him, went away. Harun rose to honor him; he kissed him on the forehead. Then he turned to his children and said to them: “Go before your uncle and master; take the stirrup; arrange his clothes; and accompany him to his house.” They set out to serve the Imam. Meanwhile, the Imam, peace be on him, confided a secret to al-Ma’mun. He gave him good news of that he would be a caliph. He asked him to be kind to his children. After they had served the Imam and accompanied him to his house, al-Ma’mun said: “I was the boldest of the children of my father (in asking him).” When the people scattered, I (al-Ma’mun) asked him:
-O Commander of the faithful, (a man came in). You magnified (the man), rose for him from your gathering, received him, seated him in front of the gathering, sat down behind him, and then you ordered us to take the stirrup on his behalf. Who was that man?
-This is the Imam of the people, the Proof of Allah's mercy to His creation (Hajjatullah) and His caliph among His servants.
- O Commander of the faithful, do these qualities not belong to you? Are they not in you?
-I am outwardly the Imam of the masses by force and through oppression, while Musa b. Ja‘far is the Imam in truth. And surely he, by Allah, is more worthy of being the successor of the Messenger of Allah as the caliph than I am and anyone else among all the people. And by Allah, If you yourself attempt to take such caliphate from me, I shall take it away from you even if that means gouging your eyes, for power is blind.
Harun remained in Yethrib (Medina) for some days. When he intended to leave, he ordered a little amount of money to be given to the Imam; the amount was two hundred dinars. He asked al-Fedl b. al-Rabi‘ to apologize to the Imam on his behalf. Al-Ma‘mun was astonished at the little gift his father gave to the Imam in spite of his magnifying and respecting him. He said to him: “O Commander of the faithful, you gave five thousand dinars to the children of the Emigrants and of the Ansar, all Quraysh, the Banu Hashim, and those who had unknown lineage. You magnified and respected Musa b. Ja‘far; nevertheless, you gave to him two hundred dinars which was the meanest gift you gave to any of the people.”
Harun became displeased with al-Ma’mun and shouted at him, saying: “Silence! May you have no mother! If I gave this (Imam Musa) what I had promised, he would hit me in the face with one hundred thousand swords of his followers. The poverty of this and of his household is safer to me and you than making their hands open.”( Al-Bihar, vol. 11, p. 270-272.)




Reference : THE LIFE OF IMAM MUSA BIN JA'FAR AL-KAZIM