Board Thread:Naruto Discussions/@comment-24824859-20150716001406/@comment-4912646-20150716034228

Thing is, Kabuto didn't even know that Madara survived his battle with Hashirama. He said he suspected Madara survived and took Hashirama's power for himself, but if Madara had been reincarnated with Hashirama's cells in his chest, Kabuto's suspicions would have already been confirmed to him, wouldn't they? Though even if he didn't have any Hashirama cells, it's possible he still retained access to the Rinnegan he awakened in his old age. That part is really open to interpretation.