Who recommended Dr. Wood to Mr. Johnson?

W: Good morning, this is Dr. Wood’s office, can I help you?
M: Good morning, I’d like to make an appointment with Dr. Wood to have my teeth checked. I’d like it to be done this week if possible.
W: I’ll see whether the doctor has time to see you this week. Have you seen Dr. Wood before?
M: No, I’ve just moved to this city and a colleague of mine advised me to see him. My name is David Johnson.
W: The doctor’s schedule is full this week, Mr. Johnson. If you are having trouble with your teeth, I’ll work you in, otherwise I can give you an appointment next Friday, the 29th. Will that be all right?
M: I’m sorry. I’ll be working on the 29th. I’m not having any trouble with my teeth, but I had hoped to have the appointment this week because I won’t start work at my new job until next Monday.
W: Are you free on Saturday, the 23rd at 10:00 a.m.?
M: That will be fine. Thank you.
W: We’ll see you on the 23rd then Mr. Johnson, goodbye.
M: Goodbye.