In [1, p. 62 exercise 3.12] we are asked to prove that for a WSS random process
 $r_{xx}[-k]=r^{\ast}[k].$ (1)

Solution: Using the definition of the autocorrelation [1, eq. (3.40,p52)]:
 $r_{xx}^{\ast}[k]$ $=$ $E\left\{x[n+k]x[n]^{\ast}\right\}^{\ast}$ $=$ $E\left\{x[n+k]^{\ast}x[n] \right\}$ $=$ $r_{xx}[-k]$

QED.

[1] Steven M. Kay: “Modern Spectral Estimation – Theory and Applications”, Prentice Hall, ISBN: 0-13-598582-X.