Feeling like husband is cheating

Does anyone know of any apps I can Dowload to check his messages to see if my spouse is cheating ? I can't get to his phone so it would need to be without him knowing about it. I've asked n tried talking to him but he says no and I cannot get rid of this horrible feelings and the signs ive been seeing...I just want to know once and for all so I can move on with my life! Please help....