In the subject, you specifically said 'C', but in the actual text of the post you just said "source"

Assembler is a low-level language (LLL) and, as such, gives a 1:1 mapping from source instructions to machine instructions.
Therefore, provided you can identify which bytes are instructions, conversion back to Assembler is easy.
However, it's the identifying of the instruction bytes which, as has already been said, is the difficult bit!

'C', on the other hand, is a high-level language; a single 'C' source statement could produce anything from none to several dozen instructions!
With modern optimising compilers it can be hard enough to follow the generated assembler even when you do have the original source in front of you!!
Therefore it is extremely unlikely that you will find anything which will give any sort of worthwhile 'C' source from a binary object! :-(

If you really, really, really, really do absolutely have to do this "reverse engineering", I'm afraid you're going to have to do it in assembler! :-(