Commentary for Chagigah 6:1

And who is like Your people, Israel, one nation in the land - It is stated in the Midrash "three testify on each other: Israel, the Shabbat, and the Holy One. Israel and the Holy One attest to the Shabbat as a day of rest. Israel and Shabbat attest to the Holy One that God is One. The Holy Blessed One and the Shabbat attest to Israel - that they are singular among the nations. And we rely on this to say "You are One [and Your Name is One and who is like Israel, one nation in the world]" in the Shabbat afternoon prayer even though this is not really specifying the day of Shabbat, as is done in the evening and morning prayers.