Mandatum novum do vobis: ut diligatis invicem,
sicut delexi vos, dicit Dominus.
Ps. Beati immaculate iin via: qui ambulant in lege Domini.
A new commandment I give unto you: That you love one another,
as I have loved you, says the Lord.
Ps. Blessed are the undefiled in the way: who walk in the law of the Lord.
A new commandment.
[Antiphon for the Maundy or Washing of Feet]
Here is a video of featuring the traditional plainsong setting of the antiphon, and showing the washing of feet during what appears to be a solemn Usus Recentior Mass (note the concelebrants).
While looking for this, I also came across this modern setting, which I rather like.