Reference for Shevuot 63:16

אמר רב חסדא הא מני ר' יוסי הגלילי היא דאמר אפשר לצמצם

Hence, we deduce from this, the oath [must be uttered] outside the Beth Din, and denial [must be] before the Beth Din. IF THEY BOTH DENIED IT TOGETHER, THEY ARE BOTH LIABLE.

Jastrow