Reference for Niddah 10:21

אומרים לו שנה ושונה

Now, however, that R. Johanan ruled that in the case of doubtful uncleanness conveyed through a human agency<span class="x" onmousemove="('comment',' As in that of the bed and the menstruant during the twenty-four hours preceding the observation of the flow or in that of a dead creeping thing that was carried by a man and a doubt arose as to whether it came in contact with a certain clean object. ');">²⁰</span>

Jastrow