Board Thread:Naruto Discussions/@comment-24098614-20141206030114/@comment-5407586-20141207105806

The only known places where one can learn senjutsu are Mt. Myoboku and Ryuchi Cave. Minato and Naruto, who both learned from the Toads, have the same markings (Jiraiya too, however his inability to perfectly balance natural energy gave him a toad-like appearance); Kabuto (Dragon Cave) and Hashirama have different markings. This is enough evidence to prove that the two of them learned senjutsu in different places. Hashirama might have learned it in the Shikkotsu Forest, seeing that's a place as famous as Mt. Myoboku and Ryuchi Cave, though there's no proof for that.