Aligning equality signs while using stack relations

Question

I am working with an align environment and would like to add some hints at the equality signs by specifying the used equation number at some steps. However, if I do that the equality signs don't align anymore. Is there a way to get the equality signs to align instead of the whole stack relation? See below for a minimal example.

\documentclass{book}
\usepackage{amsmath}
\begin{document}
\chapter{Test align equality}
\begin{equation}\label{eq::ac}
    a=c
  \end{equation}
\begin{align}
    a &= b \
    &\stackrel{(\ref{eq::ac})}{=} c
  \end{align}
\end{document}

Answer 1

You can have it using the \mathclap command from mathtools (which loads amsmath), but I propose another solution without \stackrel, which looks better, in my opinion:

\documentclass{book}
\usepackage{mathtools}
\begin{document}
\chapter{Test align equality}
\begin{equation}\label{eq::ac}
    a=c
  \end{equation}
\begin{align}
    a &= b \
    &\stackrel{\mathclap{\eqref{eq::ac}}}{=} c
  \end{align}
\begin{align}
    a &= b \
    &= c\rlap{ \qquad by \eqref{eq::ac}}
  \end{align}
\end{document}