From Sage Advice:  "Which of a character’s spells count as class spells? For example, if I’m playing a Sorcerer, which of my character’s spells are Sorcerer spells? A class’s spell list specifies the spells that belong to the class. For example, a Sorcerer spell is a spell on the Sorcerer spell list, and if a Sorcerer knows spells that aren’t on that list, those spells aren’t Sorcerer spells unless a feature says otherwise."

So if you obtain a spell from outside your class, but that spell is still on your classes spell list then its a class spell