During the Last Supper, Christ and his disciples celebrated the Passover meal, which traditionally includes the singing of the Hallel, Psalms 113-118. When Matthew records that "after singing a hymn, they went out to the Mount of Olives," that hymn is understood to be from the Hallel. New Song, a music ministry group from Geneva College, performs Psalms 113-118 from The Book of Psalms for Worship.
You can download the lyrics from Hallel here.
You can download the lyrics from Hallel here.