תִּתֵּן אֱמֶת לְיַעֲקֹב חֶסֶד לְאַבְרָהָם אֲשֶׁר נִשְׁבַּעְתָּ לַאֲבֹתֵינוּ מִימֵי קֶדֶם
You will grant truth to Jacob, mercy to Abraham, as You swore to our fathers from the days of old.
Where did G-d promise that? Rashi says at the Akeidah (the binding of Isaac). There, the Torah recounts, “…’I swear by Myself,’ says Hashem, ‘because you have done this thing and have not withheld your son’…” (Genesis 22:16). This verse is recited as part of U’va l’Tziyon.