Infinitude of Carmichael numbers congruent to $a \pmod d$

Question

Show that if $a$ and $d$ are relatively prime, then there are infinitely many Carmichael Numbers to $a \pmod d$. (i.e. composite numbers satisfying Fermat's Little theorem, $a^{n-1} = 1 \pmod n$ if $a$ is relatively prime to $n$) I recall reading somewhere that this was proven but then again I am not sure. Any help? Thanks.