Albert Barnes on Matthew 26:30

And when they had sung an hymn. The passover was observed by the Jews by singing, or chanting, the 113th, 114th, 115th, 116th, 117th, and 118th psalms. These they divided into two parts. The 118th and 114th psalms they sung during the observance of the passover, and the others at the close. There can be no doubt that our Saviour, and the apostles also, used the same psalms in their observance of the passover. The word rendered sung a hymn, is a particle, literally meaning hymning, not confined to a single hymn, but admitting many.

Mount of Olives. Mt 21:1.

(1) "hymn" or, "Psalm"