Reference for Bava Metzia 194:1

בשעה שהיתה שאולה מתה והלה אומר איני יודע חייב

OR DURING THE HOUR FOR WHICH IT WAS BORROWED; AND THE OTHER REPLIES, 'I DO NOT KNOW', HE MUST PAY. IF THE HIRER ASSERTS: THE HIRED ONE DIED, [OR], IT DIED ON THE DAY WHEN IT WAS HIRED, OR IT DIED DURING THE HOUR FOR WHICH IT WAS HIRED, AND THE OTHER REPLIES, 'I DO NOT KNOW,' HE IS NOT LIABLE. BUT IF ONE ASSERTS THAT IT WAS THE BORROWED ONE AND THE OTHER THAT IT WAS THE HIRED ONE, THE HIRER MUST SWEAR THAT THE HIRED ONE DIED [WHICH FREES HIM FROM LIABILITY]. IF THE ONE SAYS, 'I DO NOT KNOW,' AND THE OTHER SAYS, 'I DO NOT KNOW,' THEY MUST DIVIDE.<span class="x" onmousemove="('comment',' I.e., share the loss. ');">¹</span>

Jastrow