This is documented in the Fedora documentation. From the version table there, RHEL 7 is based on Fedora 19 with some changes taken from later versions.

The relationship is quite a bit more complex than a simple table can represent. In particular, the RHEL kernel receives many updates and backports throughout its life, which Fedora kernels don’t (the kernels in supported versions of Fedora are continually updated to the latest upstream kernel).