I heard a story that goes roughly like this: after the death of his Rebbi, the Chasam Sofer decided to make Rav Nosson Adler his new Rebbi. He therefore undertook a long voyage to meet Rav Adler. On arriving, Rav Adler commented that the Chasam Sofer appeared to have a particular glow on his face. Rav Sofer replied that he was correct; on the way there had he had met and killed an Amalekite, and therefore he was filled with the happiness of the mitzva.

Does anyone have a source for this story?