so i tried a few permutations
by thine words shall thou be known
by thine words wilt thou be known
by thine deeds shalt thou be known

By your deeds you will be known
Time will tell truth will show
As we exhale every breath
We all got closer to our death

i got a couple of other non-maiden-related hits in this sort of vein:
...I guess the other answer I would give you is that despite whether or not he has specifically said his position on South Africa or choice or other issues that I was always raised by the adage that by your deeds you will be known. And I think we have to look at not just the words and speeches but his conduct. ...

by their own words shall they be known gave me a couple thousand more hits, mainly stuff like this:
http://weblog.burningbird.net/fires/000916.htm and some really random sightings like:
..."By their own words shall they be known"- a recent discussion among obstetricians helps birthing women understand their view of cesarean, VBAC and birth in general. ...

so, this does seem to be a fairly well known form, can't be fagged sourcing it though. take your pick as to the original, i'm going to bed, my beloved has the superbug and i'm back on super hygiene alert.