Gevurot Meyruach Hakodesh 13:2-3

They were ministering to Adonoi and under a tzom when the Ruach Hakodesh said to them, Set apart for Me Bar-Nabba and Sha'ul for the avodas kodesh ministry to which I have called them. Then, having continued the tzom and having davened and having laid their hands upon them, they sent them off.