Musar sur Rois 2 3:15

The Divine Presence does not rest upon any one unless there is joy (Shabbath 30b). And all of the prophets did not prophesy at such times as they desired, but they would prepare their minds and sit joyous and glad of heart and then prophesy, for Prophecy does not dwell where idleness or sadness is found, but only where joy is. Therefore, the pupils of the Prophets would place before them lyre, drum and harp and sought by their means the gift of Prophecy, as it is written "And it came to pass, when the minstrel played, that the hand of the Lord came upon him" (II Kings 3:15).