The numbered psalm that begins the Hallel prayer sequence associated with Passover is Psalm 113.
Hallel, meaning “praise,” is the liturgical group of Psalms 113 through 118. These psalms are recited on joyous occasions, especially the Three Pilgrimage Festivals: Passover, Shavuot and Sukkot. They are also used on Hanukkah and Rosh Chodesh in various forms. Psalm 113 is therefore the opening psalm of the standard Hallel sequence.
At the Passover Seder, Hallel is divided around the meal. Psalm 113 and Psalm 114 are traditionally recited before the meal, while the remaining portion follows Birkat Hamazon, the Grace After Meals. The arrangement reflects an ancient rabbinic debate between the schools of Hillel and Shammai; the practice follows the view associated with Hillel.
Readers sometimes confuse Hallel with the “Great Hallel,” usually Psalm 136, which is recited later at the Seder. Another distinction is between Full Hallel and Partial Hallel: the latter omits sections and is used on several later days of Passover, while Psalm 113 remains the sequence's opening point.