From the mass readings for The Last Supper:
"He loved his own in the world and he loved them to the end. . . .
So when he had washed their feet and put his garments back on and reclined at table again, he said to them, “Do you realize what I have done for you? You call me ‘teacher’ and ‘master,’ and rightly so, for indeed I am. If I, therefore, the master and teacher, have washed your feet, you ought to wash one another’s feet. I have given you a model to follow, so that as I have done for you, you should also do."