Musnad Ishaq

Benefits:
From the aforementioned hadith, it is understood that fasting should be commenced upon sighting the moon, and fasting should also be concluded upon sighting the moon. However, if on the twenty-ninth day of Sha'ban the sky is overcast and the moon of Ramadan is not sighted, then one should not fast. Rather, the thirty days of Sha'ban should be completed. Similarly, if on the twenty-ninth day of Ramadan the sky is overcast and the moon of Shawwal is not sighted, then the fasts should be completed as thirty, because it is not permissible to fast on a doubtful day. That is, if it is the thirtieth day of Sha'ban, the sky is overcast that night, and due to this the moon cannot be sighted, then certainly there will be doubt as to whether the next morning is Ramadan or the thirtieth of Sha'ban. Therefore, one should not fast, as it is narrated from Sayyiduna Ammar bin Yasir radi Allahu anhu that whoever fasts on a doubtful day has disobeyed Abul Qasim sallallahu alayhi wa sallam. (Sunan Abi Dawud, no. 2334; Sunan Darimi, no. 1682, its chain is hasan)