Reference for Yevamot 240:10

והאמר רבה בר בר חנה לדידי חזי לי ההוא טייעא דשקיל ספסירא וגיידיה לגמליה ולא אפסיקתיה לנערותיה אמר אביי ההיא כחישא הויא

was lowered into the sea and Only his leg was brought up,<span class="x" onmousemove="('comment',' Lit., 'and it did not go up in their hands but his leg'. ');">²⁵</span> and the Sages ruled: [If the recovered leg contained the part] above the knee [the man's wife] may marry again,<span class="x" onmousemove="('comment',' Since after the loss of so much of the limb the man cannot survive. ');">²⁶</span> [but if it contained only the part] below the knee she may not remarry!<span class="x" onmousemove="('comment',' Because a man may survive even in such circumstances. The drowning also cannot be regarded as a certainty since the waters may have thrown the body up on another shore where the man's life may have been saved. Now, if our Mishnah represents the view of R. Simeon b. Eleazar, remarriage should be forbidden even in the case where 'the part above the knee' was also torn away! ');">²⁷</span>

Jastrow