My bf says he doesnt talk to his ex gfs but I saw his phone one day and his ex texted him. what do i say?

I saw his phone light up on the counter one day so glanced at it from across the room and saw his exes name. At the time I didnt know it was his ex. Now I know. I didnt even touch his phone so I didnt read his text message. How do I let him know that I know she still texts him. I dont want him to think I'm going through his phone bc I'm not.