Nasai

(1) The aforementioned narration has been declared weak in its chain of transmission by the researcher of the book, whereas other scholars have deemed it authentic based on supporting evidences. From the discussion among the scholars, the preferred opinion appears to be that, despite the narration being weak in its chain, it becomes actionable due to other supporting evidences. And Allah knows best! For further details, see: (Dhakheerat al-‘Uqba Sharh Sunan an-Nasa’i: 21/31-38)

(2) By “the day of doubt” is meant the thirtieth day of Sha’ban, because on this day there is a possibility that it might be the first day of Ramadan. Some people, out of caution, fast on this day without sighting the crescent, thinking perhaps the crescent has appeared. However, this caution is actually disobedience to the true Shari’ah. (For further details, see Hadith: 2118-2127)

(3) “Abul Qasim” is the kunyah (agnomen) of the Messenger of Allah (sallallahu alayhi wa sallam). Sometimes the noble Companions (radi Allahu anhum) would address him by this kunyah instead of his name. Generally, they would refer to him with exalted titles such as “Messenger of Allah” and “Prophet of Allah” (sallallahu alayhi wa sallam), etc.

(4) “He has disobeyed Abul Qasim (sallallahu alayhi wa sallam).” Any narration in which a Companion utters such words is, in ruling, considered marfu‘ (attributed directly to the Prophet).

1:
"That in which there is doubt" refers to the 30th day of Sha'ban, meaning that due to clouds, the crescent was not sighted on the 29th day, so someone fasts with the thought that perhaps this is either the 30th day of Sha'ban or the first day of Ramadan—lest it actually be Ramadan. In this way, fasting on a day of doubt is not correct.

Benefits and Issues:
➊ The "day of doubt" refers to the situation where it is not known whether the new moon has been sighted today or not.
➋ The meaning of this narration is established by authentic narrations; for this reason, some scholars have declared this narration to be authentic.

Lexical Explanation 528:
أَلیَومَ الَّذِی یُشَّکُّ فِیْہِ (the day about which there is doubt): "یُشَّکُّ" is in the passive form. This refers to the thirtieth day of Sha'ban, when due to cloudiness or similar reasons the moon is not sighted that night, and thus doubt arises as to whether it is Ramadan or still Sha'ban.

Benefits and Issues 528:
The aforementioned narration has been declared weak in chain (sanad) by our esteemed researcher, while other scholars have considered it authentic due to supporting evidences (shawahid). Furthermore, Imam Bukhari rahimahullah has also mentioned it in a suspended (mu'allaq) form, and from other proofs as well, the prohibition of fasting on the day of doubt is established. Therefore, despite the aforementioned narration being weak in chain, it is, due to other supporting evidences and corroborations, actionable and a valid proof. And Allah knows best. For further details, see: [أرواء الغليل : 127-125/4 ، رقم : 961]
2. Islamic Shari'ah has clearly established this principle: begin fasting upon sighting the crescent, and likewise end the fasts upon sighting the crescent of Eid. Now, if the crescent is not sighted on the night of the twenty-ninth of Sha'ban, then fasting the next morning, due to it being doubtful, is prohibited. The opinions of experts in astronomy may be correct to some extent, but they cannot be considered obligatory and definitive.