Reference for Shevuot 63:11

מנלן דאכפירה בב"ד הוא דמחייבי אחוץ לב"ד לא מחייבי

It was taught in agreement with Samuel: If they saw him coming after them, and said to him: 'Why are you coming after us? We swear we know no testimony for you', they are exempt; but in the case of a deposit, they are liable.

Jastrow