You say that you think he picked it up at daycare, so why not start there? Talk with the staff members, find out how they handle this type of behavior. Does your son hit or get hit at school? Talk with other parents, especially those of your son's friends and the parents of anyone he may not get along well with. Also ask to schedule a time when you can go observe your child interacting with other children.
Also, talk with your son. Try to find out who he has seen getting hit.
If you can find the root of the.problem, it will be a great place to start to try fixing it.
Also... Is there certain triggers that seem to cause him to start hitting you? Such as, every time you tell him "No"? After certain activities? When he is tired or bored?
If he hits you when he is frustrated or angry, try modeling for him a better way to express those feelings. Perhaps you can find picture type books at your local library where one character hits another, then read him the book,.