Majma al-Zawaid — Hadith 10336

Sayyiduna Jundab bin Abdullah (may Allah be pleased with him) narrates about the Prophet (peace and blessings be upon him) that he sent a group of people and appointed Sayyiduna Abu Ubaydah bin al-Jarrah (may Allah be pleased with him) as their commander. When they were about to set off, he began to weep out of love for the Prophet (peace and blessings be upon him), and he sat down. The Prophet (peace and blessings be upon him) appointed Sayyiduna Abdullah bin Jahsh (may Allah be pleased with him) as their commander in his place, wrote a document for him, and ordered him not to read that document until he reached such-and-such a place. The Prophet (peace and blessings be upon him) said: "Do not force any of your companions to march with you." When he read that document, he said Inna lillahi wa inna ilayhi raji'un (We belong to Allah and to Him we return), and then said: "Allah and His Messenger will be heard and obeyed." Then he informed his companions of the situation and read that document out to them, whereupon two men went back and the rest went on. These people met Ibn al-Hadrami, and they killed him. They did not realise whether that day was a day of Rajab or of Jumada al-Thani (its last day). The idolaters said to the Muslims: "You have killed in the sacred month." So Allah Almighty revealed this verse about it: "They ask you about fighting in the sacred month." One of them said: "Even if no sin has befallen them, they will not receive any reward either." So Allah Almighty revealed this verse about it: "Indeed, those who believed, and who emigrated and strove in the path of Allah, those are the ones who hope for the mercy of Allah, and Allah Almighty is Forgiving, Merciful."
Al-Tabarani recorded it, and its narrators are thiqat (trustworthy).