Among the sources he referenced was a Zohar from Metzorah. It says that just as one will be accountable for using speech to bad effect, one will be held accountable for not having spoken when one could have used speech for good.
This is a profound point. We tend to think of shmiras halashon, which is guarding speech from lashon hara -- that is avoiding the negative. But how often do we consider what we can say that will be a good use of speech? The Zohar tells us that it is not enough to not do bad. One can't be complacent about what one has avoided; one has to actively do good.