Questioner: When someone fasts Mondays and Thursdays, is that better or the fast of Dawud?
Al-Albaani: What do you mean by better?
Questioner: Better, superior, which is better?
Al-Albaani: Is what better?
Questioner: Fasting Mondays and Thursdays or the Fast of Dawud [which was to fast one day on and one day off].
Al-Albaani: No, the Fast of Dawud [is better], do you have textual proof that fasting on Mondays and Thursdays is better?
Al-Albaani: Thus, an invalid question, since you don’t have a text which states that fasting on Mondays and Thursdays is better but, on the contrary, there is a text stating the opposite of that.
The sum of my research comes to the conclusion that it is possible that we, the followers of Muhammad ﷺ, [it is possible that] when we fast one day on and one day off, that at the same time as such a fast being the Fast of Dawud it is also possible that it is better than the Fast of Dawud ﷺ, how so?
Because it is possible that in Dawud’s legislation ﷺ there wasn’t a prohibition against fasting four days of Eed, in fact it is possible that he never had Eed al-Adhaa or Eed al-Fitr, and maybe he never had the prohibition against fasting on Fridays—we have such prohibitions, so whoever combines implementing what he has been ordered with and stopping at what he has been prohibited from is better than a person who limits himself to doing what he has been ordered with alone.
And so we, when we add to this method of fasting one day on and one day off, which is the Fast of Dawud, [when to that we add] keeping away from the days which have been forbidden [to fast], like Fridays and Saturdays, our fast will be better than continuing to fast one day on and one day off.
And if the fasting day falls on a Friday and it had not been preceded with a [fast on] Thursday, this would not be better, i.e., continuing to fast according to the madhhab of Dawud ﷺ [would not be better here, because you can’t fast a Friday on its own], what is better is to proceed to fast according to the madhhab of Muhammad ﷺ in which there are orders and prohibitions, so whatever we are ordered with, we follow, and whatever we are forbidden, we refrain from.
Al-Hudaa wan-Noor, 144.