“‘Peace be on you, Apostle of Allah! Peace be on you, father!”

The narrators have said: “One of the reasons which moved Hārūn to imprison the Imām Kazim (as) is that he (Hārūn) ¾surrounded by the nobles, eminent figures, ministers, and high-ranking officials of his state ¾went to pay a visitation to the tomb of the Prophet, may Allah bless him and his family, saying: ‘Peace be on you, Apostle of Allah! Peace be on you, cousin.’”
He was seeking to show his proud position over the others by his blood relation with the Prophet, may Allah bless him and his family, for he undertook the office of the caliphate through this means. As for Imām Mūsā al-Kāzim, he was standing beside him. He greeted the great Prophet, saying: “‘Peace be on you, Apostle of Allah! Peace be on you, father!” Hārūn al-Rashid lost his mind. The expression on his face changed and the anger in it became transparently obvious. That is because the Imām was closer than him to the Prophet and more adjacent to him than others. So the tyrant, Hārūn, became angry and asked the Imām: “Why did you say that you were closer to Allah’s Apostle, may Allah bless him and his family, than me?”
The Imām gave to him a definite, undeniable proof, saying: “If Allah’s Messenger, may Allah bless him and his Household, was resurrected and asked you for your daughter’s hand, would you respond to him?”
“Glory belongs to Allah,” retorted Hārūn, “I seek to show my proud position over the Arabs and non-Arabs through this (deed).”
The Imām, peace be on him, continued to establish proofs against him regarding his being closer to the Prophet, may Allah bless him and his family, than him, saying: “However, Allah’s Messenger does not ask me for my daughter’s hand; nor do I marry (her) to him, for he is our father, not your father; therefore, I am nearer to him than you.”
Then the Imām established another proof against Hārūn’s words, saying: “Is it permitted for Allah’s Apostle, may Allah bless him and his family, to visit your womenfolk while they are unveiled?”
“No,” Hārūn replied.
The Imām explained: “However, he has the right to visit my womenfolk, and it is permitted for him to do that; therefore, I am nearer to him than you. ( Hayāt al-Imām Mūsā Bin Ja'far, vol. 1, pp. 161-162. )


Reference :The life of Imām'Ali Bin Mūsā al-Ridā