Musnad Abu Ya'la al-Mawsili 65:16

'Abd al-A'la ibn Hammad narrated to us, Mu'tamir ibn Sulayman narrated to us, he said: I heard my father from a companion of his, and he is al-Hadrami, from Abu al-Siwar, narrating from Jundub ibn 'Abd Allah: That the Messenger of Allah (ﷺ) sent a group and appointed Abu 'Ubaydah ibn al-Jarrah over them. When he began to depart, he wept out of longing for the Messenger of Allah (ﷺ). So he sent a man in his place called 'Abd Allah ibn Jahsh and wrote a letter for him, and commanded him not to force any of his companions to travel with him. When he read the letter, he said, "We belong to Allah and to Him we shall return," and said, "Hearing and obedience," meaning to Allah and His Messenger. He informed them of the news and read the letter to them. Two men returned, and the rest of them proceeded. They encountered Ibn al-Hadrami and killed him, and it was not known that day whether it was Rajab or Jumada. The polytheists said to the Muslims, "You have done such and such in the sacred month." They came to the Messenger of Allah (ﷺ) and told him the story. Then Allah revealed: "They ask you about the sacred month—fighting therein. Say, 'Fighting therein is great...'" up to His saying, "...and fitnah (disbelief/polytheism) is greater than killing." (Al-Baqarah). He said: "Polytheism." Some of those who were in the expedition said, "By Allah, only one person killed him. If it is a good deed, then he has taken charge of it, and if it is a sin, then he has committed it." Some of the Muslims said, "If they have not incurred a sin in this month of theirs, then they have no reward in it." Then Allah revealed: "Indeed, those who have believed and those who have emigrated and fought in the cause of Allah—those expect the mercy of Allah. And Allah is Forgiving, Merciful." (Al-Baqarah).