Reference for Sanhedrin 152:14
וכבש עליו לתוך המים: רישא רבותא קמ"ל וסיפא רבותא קמ"ל רישא רבותא קמ"ל אף על גב דלאו איהו דחפו כיון דאין יכול לעלות משם ומת חייב סיפא רבותא קמ"ל אע"ג דדחפו כיון דיכול לעלות משם ומת פטור
<b><i>MISHNAH</i></b>. THE FOLLOWING ARE DECAPITATED: A MURDERER, AND THE INHABITANTS OF A SEDUCED CITY. A MURDERER WHO SLEW HIS FELLOW WITH A STONE OR AN IRON, OR KEPT HIM DOWN UNDER WATER OR IN FIRE, SO THAT HE COULD NOT ASCEND THENCE, IS EXECUTED. IF HE PUSHED HIM INTO WATER OR FIRE, BUT SO THAT HE COULD ASCEND, YET HE DIED, HE IS FREE [FROM DEATH]. IF HE SET ON A DOG OR A SNAKE AGAINST HIM [AND THEY KILLED HIM], HE IS FREE FROM DEATH. BUT IF HE CAUSED A SNAKE TO BITE HIM [BY PUTTING HIS JAWS AGAINST HIM] — R. JUDAH RULED THAT HE IS EXECUTED; THE SAGES, THAT HE IS NOT.