Harry: Before, down by the lake when I was with Sirius, I did see someone. That someone made the Dementors go away.
Hermione: With a patronus? I heard Snape telling Dumbledore. According to him, only a really powerful wizard could've conjured it.
Harry: It was my dad. It was my dad who conjured the patronus.
Hermione: But Harry, your dad's...
Harry: Dead, I know. I'm just telling you what I saw.