Question: If it is cloudy or overcast, is it obligatory or prescribed to observe Sawm (Fasting) on the Day of Doubt (the 30th of Sha‘ban) as a precaution, in case the month has begun?
Answer: It is not permissible to perform Sawm on the Day of Doubt (so called because there is doubt concerning it – is it the last day of Sha’ban or the first day of Ramadan), even if the sky is cloudy, because the Messenger (peace be upon him) said: Start Sawm (Fast) when you see it (the new moon of Ramadan) and stop Sawm when you see it (the new moon of Shawwal); but if the sky is overcast (and you cannot see it), complete the number of days (of Sha‘ban to thirty days). He (peace be upon him) also said: Do not observe Sawm for a day or two days ahead of Ramadan except a person who is in the habit of observing a particular Sawm; they may fast on that day.
As for what is narrated about Ibn ‘Umar (may Allah be pleased with him), that he used to observe Sawm on the 30th of Sha‘ban if it was cloudy, this was his Ijtihad (juristic effort to infer expert legal rulings), but the correct opinion is the opposite of this; it is obligatory not to observe Sawm on that day. Ibn ‘Umar practiced Ijtihad in this case, but his Ijtihad opposed the Sunnah (whatever is reported from the Prophet), may Allah forgive him!
The correct opinion is that the Muslims must not observe Sawm on the 30th of Sha‘ban, if the new moon of Ramadan is not sighted. If it is cloudy, it is obligatory not to observe Sawm. It is not permissible to start observing Sawm until the sighting of the new moon is confirmed or Sha‘ban is completed with thirty days. This is what is obligatory on Muslims, and it is not permissible to oppose the Nas (Islamic text from the Qur’an or the Sunnah) for the personal opinion of an individual, whether he is Ibn ‘Umar or anyone else, because the Nas takes precedence over all opinions, as Allah (Glorified and Exalted be He) says: And whatsoever the Messenger (Muhammad peace be upon him) gives you, take it; and whatsoever he forbids you, abstain (from it). He (Glorified and Exalted be He) also says: And let those who oppose the Messenger’s (Muhammad peace be upon him) commandment (i.e. his Sunnah – legal ways – orders, acts of worship, statements, etc.) (among the sects) beware, lest some Fitnah (disbelief, trials, afflictions, earthquakes, killing, overpowered by a tyrant) should befall them or a painful torment be inflicted on them.
Source: Fatwas of Ibn Baz