reference request: connection between automorphs of quadratic forms and real quadratic number field

Question

Let $Q$ be a primitive binary quadratic form. There is a relation between the generator of the group of integral automorphs and the fundamental unit of the corresponding real quadratic field. This is described in Remark 2.6 on page 6 of the following paper https://arxiv.org/pdf/0804.0725.pdf This is mentioned without a reference here... Can someone know a reference for this by any chance? Thank you.