Board Thread:Theories and Speculation/@comment-1872728-20160229033316/@comment-1512913-20160319185003

It's in the Obito flashback, after he massacres Rin's pursuers and becomes 'Madara'... Zetsu introduces Obito to Nagato, Konan and Yahiko and Zetsu states that Nagato is of Senju lineage.