When did Dumbledore use the Patronus as a signal?

[jmjklj]

No: HPFGUIDX 136969

 When Tonks sends her Patronus as a signal in Ch. 8 of HBP, here is 
Harry's response:  "Was that a Patronus?" asked Harry, who had seen 
Dumbledore send messages like this.

My question is: Exactly when did we see this?  I know on JKR's site in 
the FAQ poll she answered that we had already seen this in GOF.  I 
don't recall any scene where that was used.  Could someone refresh  my 
memory?

Thanks in advance.

-Kathryn