Narrated Aishah, Ummul Muminin: The Messenger of Allah ﷺ used to count the days in Sha'ban in a manner he did not count any other month; then he fasted when he sighted the new moon of Ramadan; but if the weather was cloudy he counted thirty days and then fasted.
Hadith Referenceسنن ابي داود / كتاب الصيام / 2325
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:إسناده صحيح, مشكوة المصابيح (1980), صححه ابن خزيمة (1910 وسنده صحيح)
Benefits and Issues: It is not permissible to fast in a situation of uncertainty. This will be counted as a day of doubt, and likewise, one should not fast with the intention of anticipating (the month), because this is prohibited.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2325