I pray for you to have peace of mind. This is the correct expression. Peace is a non-count noun.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
AlpheccaStarsI pray for you to have a peace of mind. - It might be confused with a piece of my mind, which has an entirely different meaning!I didn't realize that. I might be misunderstood that I thought the person has no brain/mind. Sorry about that.