Al-Ashtar (may Allah have mercy on him) narrates: Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) used to beat people for praying after Asr. Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) narrated: The Messenger of Allah (peace and blessings be upon him) sent me on a military detachment, and we came upon a household who had affirmed the Oneness of Allah. Sayyiduna Ammar (may Allah be pleased with him) said: "These people have protected themselves from us by their affirmation of Oneness." But I paid no attention to what Ammar said, so he said: "Listen! I will certainly tell the Messenger of Allah (peace and blessings be upon him)." When we came to the Messenger of Allah (peace and blessings be upon him), he complained to him about me. When he saw that the Prophet (peace and blessings be upon him) was not taking retribution from me, he turned away with tears flowing from his eyes. The Prophet (peace and blessings be upon him) called him back and said: "O Khalid, do not revile Ammar, for whoever reviles Ammar, Allah will revile him; whoever hates Ammar, Allah will hate him; and whoever calls Ammar foolish, Allah will declare him foolish." Khalid (may Allah be pleased with him) said: "O Messenger of Allah, seek forgiveness for me. By Allah, nothing kept me from loving him except my regarding him as foolish." Khalid (may Allah be pleased with him) said: "Of all my sins, none seems more fearful to me than my calling Ammar foolish."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3815