What do I write in this sympathy card ?

Someone I know through a mother and toddler groups one year old has tragically and unexpectedly died. I obviously want to send a card but I want to write something thoughtful but not too personal as I really didn't know him that well. I've just got no idea what to write but don't want to just sign my name. Can anyone help? Although I'm sure anything I did write would be of little comfort at this dreadful time.