Halakha Mazmur 136:1: Mishneh Torah, Leavened and Unleavened Bread

And afterwards, he [washes] his hands and recites the Grace over the Meal over a third cup and drinks it. And afterwards, he pours a fourth cup and finishes the Hallel over it. And he recites the blessing over the song, and that is "May all of your creatures praise you, etc." And he recites the blessing, "who creates the fruit of the vine," [drinks it] and does not taste anything afterwards the whole night except for water. And he should pour a fifth cup and say the Great Hallel (Psalms 136) upon it, from "Give thanks to the Lord, for He is good" (Psalms 136:1) to "Upon the rivers of Babylon" (Psalms 137:1). And this cup is not obligatory like the [other] four cups. And he can finish the Hallel in any place that he desires, even though he is not in the place of the meal.