A.T. Robertson Commentary Mark 5:8

"For he said unto him, Come forth, thou unclean spirit, out of the man." — Mark 5:8 (ASV)

For he said (ελεγεν γαρ). For he had been saying (progressive imperfect). Jesus had already repeatedly ordered the demon to come out of the man whereat the demon made his outcry to Jesus and protested. Matthew 8:29 had "before the time" (προ καιρου) and 8:31 shows that the demons did not want to go back to the abyss (την αβυσσον) right now. That was their real home, but they did not wish to return to the place of torment just now.