Simple random walk on $\mathbb{Z}^d$- average distance from the starting point

Question

So, I've found on Stackexchange that: $$\mathbb{E} \left[ \mid S_n \mid \right]=\sqrt{\frac{2n}{d}}\frac{\Gamma (\frac{d+1}{2})}{\Gamma (\frac{d}{2})},$$ but I'm not sure if I can cite it in my thesis, like straight from someone's comment...

Do you know any book that provides this formula (even without proof)?