Reference for Bava Metzia 196:1

שואל חדא להד"מ וחדא אין דשאלה מתה ואידך לא ידענא אי דשכירות מתה ודקיימא דשאלה או דשאלה מתה ודקיימא דשכירות דמתוך שאינו יכול לישבע משלם:

'As for one, the claim is entirely unfounded; with respect to the second, it is true that a borrowed one died; and as to the others, I do not know whether it was the hired one that died and the one alive is the borrowed one, or whether it was the borrowed one that died and the one alive is the hired one;' and since he cannot swear he must pay.

Jastrow