I believe, as most have said....it's Obito, but as far as Sage Mode. I believe they are actually referring to the mode that comes with becoming the Juubi Jin. I'm not entirely sure though.
what about this translate? Madara fused his attack with Sage Mode... but Obito could still reflect the attack... which means that Obito stole a littlebit of Sage Mode?
Hmm....I dont know....I think Madara is saying you were able to transform into sage mode once. Meaning his Rikudou Obito form. Then he says you even managed to steal that power....commenting on his current appearance. Just a guess.... I believe he stole it when his arm impaled Madara.