Musannaf Ibn Abi Shayba 28:510

Waki’ narrated to us, he said: Sufyan narrated to us, from Makhariq ibn Khalifah, from Tariq ibn Shihab, from Salman, who said: A man entered Paradise because of a fly, and a man entered the Fire because of a fly. Two men passed by a people who had devoted themselves to an idol of theirs, and they said: "No one shall pass by us today without offering something." They said to one of them: "Offer something." He refused, so they killed him. They said to the other: "Offer something." They said: "Offer even a fly." He said: "And what is a fly?" Then he offered a fly, so he entered the Fire. Salman said: So this one entered Paradise because of a fly, and this one entered the Fire because of a fly.