Tradition
St. Augustine
The Enarrations, or Expositions, on the Psalms
Psalm 133 › Section 1
1. This is a short Psalm, but one well known and quoted. “Behold, how good and how pleasant is it, that brethren should dwell together in unity”. So sweet is that sound, that even they who know not the Psalter, sing that verse....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)