As far as I know this is not possible without your involvement.
If you know for sure this person never contacted you on WhatsApp I suppose you have an exchange account from your job. If your jobs system administrator entered his number in the contacts, his number will automatically show up in your contacts as well.
Open the Contacts app on your phone, tap Groups in the upper left corner and disable the group for your job. If you don't see his number now in contacts, then I'm right and the system administrator of your job included his number in the group.
Don't forget the group for your job again.
07-24-2016 01:51 PM