דברים יג:יא מדרש: Sifrei Devarim

(Devarim 13:11) "And you shall stone him with stones and he shall die": I might think, with many stones; it is, therefore, written (Vayikra 24:23) "and they stoned him with a stone." If "with a stone," I would think with one stone! Say, then, if he did not die with the first, he is to be put to death with a second.