Alec Lightwood: What’s going on?

Magnus Bane: You tell me, Alexander. From the day I met you, the one thing I knew I could count on from you was honesty. Absolute, total… honesty.

Alec Lightwood: I can explain.

Magnus Bane: You knew about the sword?

Alec Lightwood: I just found out. I swear.

Magnus Bane: When? When did you know? Tonight? When I talked about trust. Did you know then?

Alec Lightwood: I did.

From Shadowhunters – Season 2 Episode 17: ‘A Dark Reflection’ (2×17)