Abu al-Salt narrates: Sayyiduna Umar (may Allah be pleased with him) wrote to us while we were with Sayyiduna al-Nu'man bin Muqarrin al-Muzani (may Allah be pleased with him). He said: "When you meet the enemy, do not flee, and when you obtain spoils, do not steal from them." The narrator says: When we met the enemy, Sayyiduna al-Nu'man (may Allah be pleased with him) said: "Give the people time." The narrator says: This was on a Friday. (Sayyiduna al-Nu'man (may Allah be pleased with him) said: Give them time) until the Commander of the Believers ascends the pulpit and prays for victory, then fight them. When Sayyiduna al-Nu'man (may Allah be pleased with him) was struck down, he said: "Cover me with a cloth, advance upon your enemy, and do not let me alarm you." The narrator says: So we advanced upon the enemy, and Allah Almighty granted us victory. When the news reached Sayyiduna Umar (may Allah be pleased with him) that al-Nu'man had been martyred, and so-and-so had been martyred, and so-and-so had been martyred, as well as men whom we do not know, Sayyiduna Umar (may Allah be pleased with him) said: "But Allah Almighty knows them."
Al-Tabarani recorded it, and its chain is hasan (good).