This is when Harry sees Dumbledore's memories in Half Blood Prince, Voldemort comes to Hogwarts to ask for a job. Dumbledore tells Voldemort he knows all about the Death Eaters in the Hog's Head. Voldemort is furious that Dumbledore knows so much and he says:
"You are as omniscient as ever, Dumbledore."
"No, no. Merely friendly with the local barmen." replies Dumbledore.
This did not mean much then, but later on, we learn that the barman is actually Aberforth Dumbledore, Albus' brother.
— Quora user Yedu Krishnan