At Terce, Sext and None on Monday let the nine remaining sections of Psalm 118 be said, three at each of these Hours.Psalm 118 having been completed, therefore, on two days, Sunday and Monday, let the nine Psalms from Psalm 119 to Psalm 127 be said at Terce, Sext and None, three at each Hour, beginning with Tuesday. And let these same Psalms be repeated every day until Sunday at the same Hours, while the arrangement of hymns, lessons and versesis kept the same on all days; and thus Prime on Sunday will always begin with Psalm 118.

I am fairly certain that Benedict's Psalm 118 is what most of us now call 119. The Greek numbering of the Psalms, used by most of the early church, is different from the Hebrew numbering of the Psalms now used by most Christians in the West.

This is the longest psalm, opening with "Happy are those whose way is blameless, who walk in the law of the Lord." It is followed by a series of shorter psalms especially well suited for Benedict's purpose.

Psalm 118 is part of the Jewish Great Prayer offered during the festivals of Passover, Weeks, and Tabernacles. It plays as especially important role in Greek Orthodox liturgy, monastic life, and the funeral service.

The Benedictines of Santo Domingo de Silos are heard chanting Psalm 121 in this Google video.