Zakariyyah Ibn Yahyah (rahimahullaah) said:

I heard Abu-Bakr Ibn Ayyaash (when) a man said to him: ”O AbuBakr! Who is the Sunni?” He said: Is the one that when the desires are mentioned (i.e. when the innovations are mentioned and criticized), he does not become bigoted (in defending or following) any of it.”

[Source: Al-Itisaam 1/114]