Chagigah 24B

אבל יד חבירו ⁠— לא. ורבי יוחנן אמר: אחד ידו ואחד יד חבירו, באותה היד. לפסול, אבל לא לטמא.

but an impure hand does not render impure the hand of another. And R' Yoḥanan said: An impure hand renders another hand impure whether it is his own hand or the hand of another, provided the second hand is touched by the same hand that came into contact with the impurity. Moreover, the impure hand affects the food it touches only to disqualify it, but not to render it impure.

ממאי ⁠— מדקתני סיפא: שהיד מטמאה חבירתה לקדש, אבל לא לתרומה. הא תו למה לי? הא תנא ליה רישא! אלא לאו שמע מינה, לאתויי יד חבירו.

The Talmud elaborates on R' Yoḥanan’s opinion. From where did he learn this? From the fact that it teaches in a latter clause in the Mishnah: For one renders its counterpart, i.e., the other hand, impure with regard to sacrificial food but not with regard to teruma. Why do I need this phrase as well? It was already taught in the first clause that one hand renders the other hand impure with regard to sacrificial food. Rather, must one not conclude from this added phrase that it comes to include the rendering impure of the hand of another as well as his own other hand?

ואף ריש לקיש הדר ביה, דאמר רבי יונה אמר רבי אמי אמר ריש לקיש: אחד ידו ואחד יד חבירו, באותה היד, לפסול אבל לא לטמא.

The Talmud observes: And Reish Lakish, too, retracted his own opinion in favor of R' Yoḥanan’s opinion. For R' Yona said that R' Ami said that Reish Lakish said: The decree that one hand renders another impure applies whether it is his own hand or the hand of another, provided the second hand is touched by the same hand that came into contact with the impurity. Moreover, the impure hand affects the food it touches only to disqualify it, but not to render it impure.