Board Thread:Naruto Discussions/@comment-26367861-20161025235033/@comment-4765188-20161026163645

It was very explicitly implied to be Hashirama's DNA Princeharris. I quote Kabuto "Yes, with my medical ninjutsu, the cells of Hashirama I've studied so far and someone's advice, I was somehow able to save his life".

OT: All you need to awaken the Rinnegan is Hagoromo's chakra. You can either get it directly from him, or recreate it by possessing both Asura and Indra's chakra. The latter takes a long time, as evidenced by Madara. With Sasuke having taken both routes, probably gave birth to his unique 6T Rinnegan.