In which sutta does the Buddha discuss his attitudes toward his audience either understanding or not understanding his teaching?

I am writing an essay on Buddhist nonattachment, in which I argue that nonattachment is compatible with a wide range of affective states and intimate relationships (contra some popular philosophical conceptions, which are greatly influenced by Stoicism). The account of Buddhist nonattachment I offer stresses an attitude of open-minded acceptance of whatever mental state arises, without attraction or aversion -- without indulging or suppressing it. In more than one place I have heard mention of a sutta in which the Buddha describes his mental states and attitudes when he teaches the Dhamma, and his audience either understands or fails to understand. From what I've heard, he says something along the lines that he is pleased when people understand the Dhamma and displeased when they do not; however, his is not overcome by those mental states. This is not a direct quote, only the impression left in my memory. Does this sound familiar? Can anyone help me find this sutta?