Hi
The formula given in reference (2) is mostly used.
Actually this depends how you define DFT and IDFT. Some people even define it by dividing formula given in Reference (2) by √N=sqrt(N).
It is only matter of choice. BUT if you use a DFT formula then see its Inverse DFT formula also. If two formula differ in scaling then their inverse will also differ by a scaling factor.
Hope this post clarifies you about DFT.