It's from a 1957 Simplicity men's robe pattern:
With dressing gown, towelling bathrobe and smoking jacket versions!! I made the dressing gown view from some JoAnn's flannel. It was a dream to sew, he loves his robe, and my 5-year-old loved picking out the fabric. I seriously recommend this pattern -- I will be making every version eventually! More photos and details on my blog!
♥ Sarah