Do you plan to kill me, Levi?
Yeah, the reason I joined the Survey Corps…
Was to kill you, Erwin.
Preview for the BD/DVD Special: “A Choice without Regrets”
[EDIT: Since you all are asking for a source, the admin of this site claims to have gotten the Blu-Ray preorder for Volume 3 and played the Blu-ray visual novels that came with it, one of which was the Demo version of “A Choice without Regrets”. Supposed to be canon backstory.]