Midras Ulangan 20:15: Mekhilta DeRabbi Yishmael & Sifrei Devarim

Once Yehudah b. Tabbai entered a ruin, where he found a man in his death throes, the knife dripping blood in the hand of the murderer — to whom he said "May '—' come upon me if neither I nor you killed him; but what can I do (to you) when the Torah has written (Devarim 20:15) 'By word of two witnesses shall a thing be established!' But the Omniscient One will exact punishment of that man (i.e., 'you')." He had hardly finished speaking before a snake bit him and he died.

(Devarim 20:15) "Thus shall you do to all the cities that are distant": The distant ones (i.e., those not of the seven nations) are subsumed in this law (to spare the women and children), but those that are near (i.e., the seven nations) are not subsumed in this law.