The verse in question comes from (1 Chronicles 29:9): “The people rejoiced in their donation, because they donated to the Lord wholeheartedly, and King David too rejoiced with great joy.” Rav Huna, quoting Rav Idi in Bereshit Rabbah 70, points out that the people were happy because their donation – their mitzvah – was successful. They gave with a full heart. And that's when something interesting happens.
David doesn’t just celebrate with them; he blesses God. The verse continues, “David blessed the Lord before the eyes of the entire congregation [and David said: Blessed is the Lord, God of Israel our father]…” But notice something. Why does he say “God of Israel our father” and not “God of Abraham, Isaac, and Israel”? It’s a subtle difference, but a powerful one. Rav Huna says it’s because David is ascribing the vow – the initial impulse to give – to the one who began with it first: Israel.
Rabbi Yehuda chimes in with another, even more direct prooftext, straight from the Torah. We find it in (Numbers 21:2), which tells us, "Israel took a vow.." But look closely at the Hebrew. It could say “vayidru”—plural, meaning all of Israel took a vow. But that's not what it says. Instead, it says “vayidar”—singular. Rabbi Yehuda argues that this is not accidental; it specifically refers to Israel the elder, Jacob himself.