Sahih Muslim Book 45, Hadith 88
A hadith like this has been narrated on the authority of Nu'man b. Bashir through another chain of transmitters.
Nu'man b. Bashir reported that Muslims are like one body of a person; if the eye is sore, the whole body aches, and if the head aches, the whole body aches.
When two persons indulge in hurling (abuses) upon one another, it would be the first one who would be the sinner so long as the oppressed does not transgress the limits.