Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) said: In the time of the Prophet (peace and blessings be upon him), Ammar (may Allah be pleased with him) reviled me, so I came to the Messenger of Allah (peace and blessings be upon him) and said: "O Messenger of Allah, were it not for you, the son of Sumayyah would not have reviled me." He (peace and blessings be upon him) said: "Steady, O Khalid! Whoever reviles Ammar, Allah will revile him, and whoever looks down on Ammar, Allah will abase him."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3819