Sayyiduna Jundub bin Abdullah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) sent out a group and appointed Ubaydah bin al-Harith (may Allah be pleased with him) as their commander. When he set out to leave, his love for the Messenger of Allah (peace and blessings be upon him) made him weep. So he (peace and blessings be upon him) appointed Abdullah bin Jahsh (may Allah be pleased with him) as commander in his place, wrote a letter and gave it to him, and told him to open and read it at such-and-such a place. And he (peace and blessings be upon him) said: "Do not force any of your companions to travel with you." When he reached the place where he (peace and blessings be upon him) had ordered him to open the letter, he opened and read it, and along with that he also said: 'Inna lillahi wa inna ilayhi raji'un' (We belong to Allah and to Him we return), and said: We hear and obey Allah and His Messenger. He said: Then one man from among his companions turned back, and the rest continued with him. They met Ibn al-Hadrami and killed him, and they did not know whether that day was in Rajab or in Jumada al-Akhirah. So the polytheists said: You have killed in the sacred month. Then Allah Almighty revealed these verses: "They ask you about fighting in the sacred month. Say: Fighting in it is a great (sin)" [Surah al-Baqarah: 217], up to this statement: "And fitnah is greater than killing" [Surah al-Baqarah: 217] - that is, they ask you (peace and blessings be upon him) about fighting in the sacred month; say: fighting in it is a major sin, but spreading fitnah is an even greater sin than killing. The narrator says: Some of the Muslims said: Even if they did a good deed, they will not receive any reward for it. So Allah Almighty revealed this verse: "Indeed, those who believed and those who emigrated and strove in the way of Allah - those hope for the mercy of Allah. And Allah is Forgiving, Merciful." [Surah al-Baqarah: 218]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17745