Question for EF Rubricists

Here is a question that some of us have been batting around for awhile.

Where in preconcilar legislation is it expressly forbidden to sing Latin propers at EF Low Mass?

I ask because many books presume that doing so is forbidden, while it is clearly permitted to sing motets, hymns, or other music, even in the vernacular, at the entrance, offertory, and communion. But I’ve yet to find an explicit citation to liturgical law to back this widely held view that it is not permitted to sing the proper texts from the Graduale Romanum. Surely such a statement from the Church is to be found somewhere, but where?