When Bad Warlocks Go Good - S1-E18
Corrected entry: After the sisters stop Brendan from killing the woman and the police arrive, the woman tells Andy that she heard the name Prue. Andy tells Prue he can place her at the scene. But she can't be the only Prue in the city and the woman never saw Prue, so he can not place her at the scene.
Correction: Andy meant that he knew Prue was there at the scene. It might not have held up in a court of law or anything, but at this time Andy knew that the sisters (especially Prue) had a habit of being at crime scenes without any reasonable explanation. And honestly, Prue isn't exactly a common name.