Israel. Not "a liturgical addition", but used to link the three Psalms of this group together (Psalms 129:1; Psalms 130:7; Psalms 130:8; Psalms 131:3). For Israel is to find rest and peace where their kings (David, and Hezekiah) found it.
Related Commentaries of Psalms 131:3
Psalms 131:3
3 Let Israel hope in the LORD from henceforth and for ever.