What is the ruling on vowing to fast on Saturday?

The Shaykh : Fine, someone else, please.

The questioner : Given that fasting on Saturday is not permissible, is it permissible for a Muslim... to vow to fast on Saturday?

The Shaykh : No, it is not permissible to intentionally do that, but if it happens to him, he must fulfill it.

The questioner : Would it be a vow in obedience to Allah?

The Shaykh : Would it be what?

The questioner : Would it be a vow in obedience to Allah if he vowed that, meaning he is obligated to fulfill it?

The Shaykh : If he vows while knowing, it is not a vow of obedience. But as you know now, without a vow, people fast on Saturday on one of the occasions we mentioned. Do we say that this fasting is fasting in disobedience for those people? We do not say to them that it is fasting in disobedience. As for us, since we know the prohibition of the Messenger (peace and blessings be upon him) regarding fasting on Saturday, it is disobedience for us. This means it falls under the issue that no Muslim has the right to impose his opinion as a binding obligation on the general Muslim community. Rather, he presents the knowledge he has, and whoever is convinced by it, so be it, and he is bound by what is required of the one who is convinced. Otherwise, he proceeds according to his previous conviction.