And when they had sung an hymn, they went out into the mount of Olives. —Mark 14:26
The Jews observed the Passover by singing or chanting the Hallel or Hallelujah. It consists of a recitation from Psalms 113-118 which is used for praise and thanksgiving. Psalms 113-114 were usually sung before the meal and Psalms 115-118 were sung at the close.
There is little doubt but that this is what Jesus and the disciples sang before departing to the mount of Olives.