Al-Ashtar (may Allah have mercy on him) narrates: Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) began speaking to us on his own without our asking him, and said: "I have not done any deed about which I fear more that it will make me enter the Fire than the matter of Ammar." We said: "O Abu Sulayman, what is it?" He said: "The Messenger of Allah (peace and blessings be upon him) sent me with some of his Companions to one of the tribes of the Arabs. I attacked them, and among them was a Muslim household. Ammar (may Allah be pleased with him) spoke to me about some people from among his companions and said: 'Release them.' I said: 'No, not until I bring them to the Messenger of Allah (peace and blessings be upon him); then, if he wishes, he may release them, and if he wishes, he may do with them whatever he wants.' I went in to the Messenger of Allah (peace and blessings be upon him), and Ammar asked permission, came in and said: 'O Messenger of Allah, have you not seen what Khalid did, this and that?' So I (Khalid) said: 'By Allah, were it not for your gathering, the son of Sumayyah would not have reviled me.' The Messenger of Allah (peace and blessings be upon him) said: 'O Ammar, go out.' So he went out weeping and said: 'The Messenger of Allah (peace and blessings be upon him) did not support me against Khalid.' Then the Messenger of Allah (peace and blessings be upon him) said to me: 'Why did you not answer the man (gently)?' I said: 'O Messenger of Allah, nothing kept me from it except my looking down on him.' The Messenger of Allah (peace and blessings be upon him) said: 'Whoever looks down on Ammar, Allah will abase him; whoever reviles Ammar, Allah will revile him; and whoever disparages Ammar, Allah will disparage him.' So I went out, followed him and spoke to him until he sought forgiveness for me."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3817