I currently have this:

with this code:

\begin{align} 0 &\stackrel{!}{=} (d_{P,f}^2)'\\ &= -2 x_p + 2x -2y_p f'(x) + \left (f(x)^2 \right )'\\ &\overset{\mathclap{

ormalfont\mbox{Chain rule}}}{=} -2 x_p + 2x -2y_p f'(x) + 2 f(x) \cdot f'(x)\\ &= -2 x_p + 2x -2y_p (2ax+b) + ((ax^2+bx+c)^2)' \end{align}

Instead of \overset{\mathclap{

ormalfont\mbox{Chain rule}}}{=} , I've also tryed \stackrel{\text{chain rule}}{=} :

The best I could come up with is \overset{\mathclap{\tiny\mbox{chain rule}}}{=} :

But it still does not look satisfying. Is it possible to make a line break within "chain rule"? Do you have a better idea for this kind of annotation?