Majma al-Zawaid — Hadith 15601

Sa'id bin Abd al-Aziz said: On the day of the Battle of Uhud, Sayyiduna Ammar bin Yasir (may Allah be pleased with him) swore an oath and the polytheists were routed, and on the day of the Battle of the Camel he swore an oath and the people of Basrah were overcome. On the day of Siffin it was said to him: It would be fitting if you swore an oath. Sayyiduna Ammar (may Allah be pleased with him) said: Even if they strike at us with their swords (that is, push us back) until they take us to the place called "Sa'afat Hajar", we would still know that we are on the truth and they are on falsehood. So Sayyiduna Ammar (may Allah be pleased with him) did not swear an oath, and he was martyred that very day. On the day of Uhud he had said: "O Jibril and O Mikal, I swear that a band of misguided people will never overcome us; indeed we are on the truth and they are ignorant people." And he went on until he broke through the ranks of the polytheists.
Al-Tabarani recorded it; it is munqati' (broken) in its chain, and its narrators are the narrators of the Sahih.