´It was narrated that:` 'Abdur-Rahman bin Zaid bin Al-Khattab addressed the people on the day concerning which there was doubt (as to whether the month had begun) and said: "I sat with the Companions of the Messenger of Allah and asked them, and they a narrated that the Messenger of Allah said: 'Fast when you see it and stop fasting when you see it, and perform the rites on that basis. If it is obscured, then complete thirty days, and if two witnesses testify then fast and stop fasting."
Explanation & Benefits
Hafiz Muhammad Ameen
(1) By “doubtful day” is meant the thirtieth day of Sha’ban, because regarding it there are both possibilities: it could be the thirtieth of Sha’ban or the first of Ramadan, especially when there was a possibility of sighting the crescent but the sky was cloudy and the crescent could not be seen.
(2) “The crescent is not sighted” due to clouds, dust, smoke, etc.
(3) “Two persons give testimony.” Two persons are necessary when the sky is completely clear, because in such a situation, the possibility of more people seeing it exists. However, if the sky is cloudy, then the testimony of even one person is sufficient, as has been mentioned in a previous hadith, because in such a situation, generally, the possibility of sighting does not exist, and even one person seeing it is considered a blessing. In this way, the hadiths can be reconciled, and reconciliation is preferable. However, for Eid, two witnesses are necessary. And Allah knows best.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2118