Bentley, who is now two, did that as a puppy. I have found that yelling, for the most part, doesn't do any good
It is a bad habit (biting or chewing on fingers) so we just took our hand away and said a firm NO. When they hear too many words I think it just confuses them. Just say no and take your hand away...maybe give him a chew stick or chew toy afterwards. Our newest pups, eight months old, caught on that it was unacdeptable really fast!