I read this story in the Shavuos edition of Sichat Hashavua:
Rabbi Bunem of Zichlin, a non-Chassid, upon drawing near to Kotzk, faltered in his Torah study. The Rebbe summoned him to convey this:
Do you know why we refer to a Torah scholar as “milei kreiso”, having filled his belly, with wisdom? Why not employ the traditional idiom of head or heart? This is because, no matter how many times one has eaten, the hunger returns daily. A Jew who learns less diligently than before for “Chassidus” can find himself another Rebbe.