有言在先

The phrase appears in Feng Menglong's late-Ming collection 《醒世恒言》 (Xǐngshì Héngyán, 'Stories to Awaken the World'): '他有言在先，你今日不须惊怕' — 'he gave prior warning, so you needn't be afraid today.' Used today to invoke a previously-stated condition or warning.

usage

Often appears at the start of a sentence to preface a condition you're about to enforce: '有言在先, ...' = 'Just so we're clear...' The implication is the listener can't claim ignorance later.