John Gill Commentary Mark 14:26

"And when they had sung a hymn, they went out unto the mount of Olives." — Mark 14:26 (ASV)

And when they had sung an hymn The Hallell, used at the passover: they went out into the Mount of Olives; Christ, and eleven of his disciples; for Judas now separated from them, and went to the chief priests to acquaint them how things were, where Jesus was going, and where they might apprehend him; (See Gill on Matthew 26:30).