Conditional Second-order Derivative and Degree of Homogeneity

I have a function $f(a,b)$ with first-order derivatives $f_a>0, f_b>0$ and second-order derivatives $f_{aa}<0, f_{bb}<0, f_{ab}>0$ .

Additionally I know that the degree of homogeneity of the function is larger than $0$ but smaller than $1$ .

In an economics paper I found the statement that given these assumptions the derivative of $f_a$ with regard to $a$ is negative if the first-order derivative with regard to $b$ is held constant:

$\frac{\partial f_a}{\partial a}\mid_{f_b=C} <0$

Why is this so?

I understand that I can decompose $\frac{f_S(a,b)}{\partial a}$ into

$f_{aa} + f_{ab} \frac{\partial b}{\partial a}$

Now, I would have to show that $f_{aa} < f_{ab} \frac{\partial b}{\partial a}$ but I don't know how to do that using $f_b=C$ and the information about the degree of homogeneity.