I am looking for a reference for the following result:
Let $A$ be an $m \times n$ real matrix. Let $\sigma(A)$ be the spectral norm of $A$. If the largest singular value of $A$ is unique, then
$$ \nabla_A \sigma (A) = \mathbf{u} \mathbf{v}^{\top}, $$
where $\mathbf{u}$ and $\mathbf{v}$ denote the first left- and right-singular (column) vectors, respectively.
I am aware of this question, and am not asking about the proof. Could you please let me know references (e.g., textbooks) about this result? Thank you.