Bannon on Twitter:
http://www.freerepublic.com/focus/f-bloggers/3619113/posts
Reportedly that is a fake Bannon account.
The “real” Breitbart.com seems to think that Bannon really said it:
http://www.breitbart.com/big-government/2018/01/03/book-steve-bannon-calls-kushner-manafort-don-jr-trump-tower-meeting-russians-treasonous/