I found this quote from a Buddhist article, which says that this verse is from Anguttara Nikaya, but did not specify the sutta. Does anyone know?
"Bhasaye jotaye dhammam,
Pagganhe Isinam dhajam,
Dhammo hi Isinam dhajo."
"Preach, glorify the Truth, and
Lift the Sage's flag on high,
Emblem of saints is goodly speech,
Truth is the flag they fly." ~Anguttara