Majma al-Zawaid — Hadith 56

Humayd bin Hilal narrates: Sayyiduna Umarah bin Qard al-Laythi (may Allah be pleased with him) took part in a military expedition. He remained there for as long as Allah willed, then he set out to return. When he drew near to al-Ahwaz and heard the sound of the adhan, he said: "By Allah! I have not prayed in congregation with the Muslims for the past three days." He headed towards the adhan, intending to offer the (congregational) prayer, but it turned out to be (the mosque of) the "Azariqah" (that is, the Khawarij). Those people said to Sayyiduna Umarah (may Allah be pleased with him): "O enemy of Allah! Why have you come?" Sayyiduna Umarah (may Allah be pleased with him) said: "Are you people not my brothers, that is, Muslims?" They said: "You are the brother of Satan; we will certainly kill you." Sayyiduna Umarah (may Allah be pleased with him) said: "Will you not be content with from me what the Messenger of Allah was content with?" They asked: "What was the thing because of which the Messenger of Allah (peace and blessings be upon him) was content with you?" Sayyiduna Umarah (may Allah be pleased with him) told them: "When I came to the Prophet (peace and blessings be upon him), I was a disbeliever. Then I testified that there is no god besides Allah Almighty and that you are indeed the Messenger of Allah, so the Prophet (peace and blessings be upon him) let me go."
(The narrator says:) But those Khawarij seized him and martyred him.
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat, and its narrators are the narrators of the Sahih.