Approach Boyfriend about cheating

So me and my boyfriend have been together over 2 years. He's had a really close friend, who is a girl, since the beginning of high school. So years. They were friends before we started dating. When me and him got together, they hung out every now and then, and its continued since then. Never heard anything bad about it... Until tonight. (This is the confusing part.) My best friend, Megan, and her boyfriend Morgan were talking. He said one of his mutual friends with my bf(Chris) told him that Chris has cheated on me with the other girl.... How do I approach him? He's never given me a reason to doubt him, but them sleeping together is possible (time wise I suppose.) How do I bring it up?