Reference for diagonalization theorems

Question

I know there is a theorem as follows: If A is a real symmetric matrix, then A is orthogonally diagonalizable; in other words, there exists an orthogonal matrix P (i.e. a real matrix satisfying $P^{-1} = P^T$ such that $P^{-1} A P$ is a diagonal matrix.

And I know how we can prove it. But I need a reference book having this theorem to which I can refer because I cannot reprove it in my paper. I just want to use it. I appreciate if anybody can introduce a reference for this to me.

Thanks

Answer 1

You can use Horn and Johnson (2013 edition), theorem 2.5.3 on page 133.