In the episode School Hard in season 2 when Angel is trying to convince Spike that he has no soul Spike sees through it and says something like "You think you can fool me? You were my sire. My yoda." In later episodes of Buffy (i forget the name of the episode but it's the episode of season 5 that Buffy gets staked and asks Spike to tell her how he beat the other slayers) Spike says Dru sired him.