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.
Explanation & Benefits
Shaykh Umar Farooq Saeedi
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