The Shaykh’s illustration: The prohibition of the Prophet (peace and blessings be upon him) against advancing Ramadan by fasting one or two days, based on his prohibition—likewise—against connecting the obligatory prayer with the voluntary prayer

The Shaykh : And in the same category, precisely, is what came in "Sahih Muslim" regarding the prohibition of the Messenger (peace and blessings be upon him) against connecting the Friday obligatory prayer with the recommended practice that follows it. He commanded separating the obligatory from the voluntary, either by speaking or by leaving and departing. The Messenger (peace and blessings be upon him) prohibited connecting the obligatory with the recommended practice that follows it. This, too, is from the category of blocking the means to harm, so that what is not part of the obligatory is not connected to it. On the day of Friday specifically, and all obligatory prayers generally, it is recommended to separate after the closing salam, either by leaving and changing location, or by speaking with your companion in ordinary conversation, in order to achieve the separation. Know that the matter is as he (peace and blessings be upon him) said regarding prayer: ; meaning, what was forbidden to you previously in prayer becomes permissible for you simply by saying "Peace be upon you and Allah's mercy" in the first closing salam. However, to emphasize this separation beyond just the salam, and from the category of blocking the means to harm so that something not part of this obligatory prayer—which is the obligatory prayer—is not connected to it, the Messenger (peace and blessings be upon him) emphasized that the one praying should speak or depart to the right, left, front, or back.
All of this confirms this matter which the Messenger (peace and blessings be upon him) commanded: not to advance before Ramadan by fasting one or two days; so that the month of Ramadan remains without any addition to it, just as it is not permissible to subtract from it. And the excess—as the common people say—is the brother of the deficiency.
He says: . Is this prohibition absolute?
The Messenger (peace and blessings be upon him) said in answer to this question: . This exception clarifies that the previous prohibition is specific to one who intentionally advances before Ramadan by fasting one or two days. As for another person who has a system of fasting—for example, if it is his custom to follow the known recommended practice of fasting Monday and Thursday of every week, and it happens that Thursday comes, and that is one day before Ramadan; does he fall under this prohibition: ?
The answer is: No, . So the one who has this custom of this prescribed fasting is permitted to advance Ramadan with such fasting; because he did not intend this advancement. And it seems the direct intent of this prohibition is fasting the day of doubt, about which a specific discussion will follow; because the one who fasts the day of doubt fasts the day that is before Ramadan, and it has not yet been established that this day—that is, the day of doubt—is from Ramadan, so he fasts it as a precaution.