Whoever fasts on the day of doubt believing that it is from Ramadan, then it turns out that that day is not…

Answer

In the name of Allah, he must convert his intention into Sha’ban. Fasting is not permitted with the intention of the month of Ramadan unless the number of Sha’ban thirty days is completed or is proven to the one in one of the ways considered to be from the month of Ramadan. Allah knows best.