Mustadrak al-Hakim — Hadith 5774

It is narrated from Sayyiduna Khalid bin al-Walid (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) sent me on an expedition, and Ammar bin Yasir was with me. We came upon some people, among whom a household had mentioned Islam. Ammar said: These people have affirmed the Oneness of Allah (tawhid). But I paid no attention to what he said, and what befell the rest of the people befell them too. The narrator said: Then Ammar began threatening me that if he saw the Messenger of Allah (peace and blessings be upon him), he would certainly tell him. So he came to the Prophet (peace and blessings be upon him) and told him the whole story. When he saw that he (peace and blessings be upon him) was not supporting him (in response to his complaint about me), he turned back weeping. Khalid said: Then he (peace and blessings be upon him) called me and said: "O Khalid! Do not revile Ammar, for whoever reviles Ammar, Allah reviles him; whoever hates Ammar, Allah hates him; and whoever regards Ammar as foolish (and contemptible), Allah makes him foolish (and contemptible)." Khalid said: O Messenger of Allah, pray for forgiveness for me. By Allah, nothing prevented me from answering him except that I considered him foolish. Khalid said: On that day there was nothing I feared more than my having called Ammar bin Yasir foolish.
The chain of this hadith is sahih (authentic), but the two Shaykhs did not record it. Mas'ud bin Sa'd al-Ju'fi and Muhammad bin Fudayl bin Ghazwan also narrated it in the same way from al-Hasan bin Ubaydullah al-Nakha'i.