What ninth-century work contains the earliest full known text of Dayenu?

The ninth-century Seder Rav Amram contains the earliest full known text of Dayenu.

Seder Rav Amram, also called Siddur Rav Amram, is a Jewish liturgical work associated with Amram Gaon, a leading Babylonian scholar of the ninth century. It is an important early witness to the developing prayer and seder traditions.

The complete Dayenu text appears in the first medieval Haggadah associated with this work. That makes Seder Rav Amram the earliest known source for the song in its full form, even though the composition of Dayenu itself may predate the surviving witness. “Earliest full known text” refers to the earliest surviving complete version, not necessarily the moment when the song was first composed.

The alternatives belong to different contexts: the Mishneh Torah is Maimonides’ legal code, the Aleppo Codex is a biblical manuscript, and Mahzor Vitry is a later medieval prayer collection.