As far as I'm aware, there is not explicit rule one way or the other - just tradition and the assumption about what words like "quorum" and "absent" mean.
Constitutionally, this falls under the "Each House may determine the Rules of its Proceedings" catchall. It does require a "quorum" to do business, and "absent" members can be compelled to attend, both of which imply physical presence, but both of which can theoretically be satisfied via telepresence as well.
The House has rule III(1), which says
Every Member shall be present
within the Hall of the House during its
sittings, unless excused or necessarily
prevented, and shall vote on each question put, unless having a direct personal or pecuniary interest in the
event of such question.
A resolution has been introduced to change this requirement, although at least a quorum of Representatives would have to gather in person in order to pass it.
As far as I can tell, the Senate rules don't address physical presence at all, although they presumably use the standard, in-person, definition of "quorum".
This Washington Post article gets into it a little deeper (before veering off into opinion), including precedents for changing the definition of quorum.