How to respond to the one who says that the prohibition of fasting on Saturday is only if it is isolated?

The Shaykh: The third opinion, which we mentioned earlier, is that the prohibition means isolating it with fasting. But we say: this is an interpretation with no basis, because we said that there is doubt regarding the authenticity of the report that the Messenger of Allah (peace and blessings be upon him) fasted on Saturday. Then, even if we accept its authenticity, it is not established that he fasted after the prohibition. Therefore, there is no basis for this interpretation. And there is no basis for such a correction. So we say: Do not fast on Saturday except in what is obligatory upon you, or unless one of you fasts it by fasting with it a day before it or a day after it.