The source, AKA where it is mentioned would be nice.
In False Gods
when Horus is shown the scattering he sees spells/prayers written on the primarchs' incubation pods. It is mentioned a couple of times in The First Heretic
if I recall correctly as well. It's also hinted at in A Thousand Sons
with regards to the Emperor and his dealings with the chaos gods. In Deliverance Lost
when the Emperor shares the secrets of the primarchs with Corax he censors a big part, which hints that he was hiding a part of their creation. Might also be mentions of it in Aurelian
, though it has been ages since I read that novella, so I might be mistaken. And there are other hints dropped throughout the series, but I am not gonna search through all the novels right now to check for all of them. Basically, it is a continuous theme throughout the HH series that the primarchs were created with the help of chaos.
Edit: Sorry, got that backwards. It was Argel Tal, during The First Heretic
I think who saw the prayers on the pods. When Horus was taken back to the scattering he was told out and out that the Emperor struck a bargain with chaos for the power to make the primarchs. But the point stands, there are continuously hints dropped in various novels that some kind of bargain was struck between the Emperor and chaos in order for the primarchs to be created. Sadly, only the Emperor on the Imperial side can genuinely confirm this, and he off course wouldn't actually admit to making deals with beings he, for centuries, tried to deny existed.