Someone said today that they saw once in one of the seforim of the Chafetz Chaim that he wrote that if someone is learning Torah (a specific topic) and the veers unnecessarily to another topic of Torah (not related to what he is learning) that it is akin to "bittul Torah".
I was somewhat surprised by this. Does anyone know the source or perhaps know a different source (outside of the writings of the Chafetz Chaim)?