The ruling on fasting Ramadhan 28 days
Question:
Is it permissible to fast only 28 days for the month of Ramadhan?
Answer:
It is confirmed in the extensive authentic Hadiths from the Messenger-Sallallahu alaihi wasallam- that the month is not less than 29 days. When it is confirmed with Islamically valid evidence that the month of Shawwal has entered after the Muslims have only fasted 28 days, this means that they did not fast the the first day of Ramadhan.Therefore they must make up for it,because it is not possible for the month to be 28 days.The month is only 29 or 30 days.
Source:
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Page no.234 Vol: 2