Ibn Majah

Benefits and Issues:

➊
The "day of doubt" refers to the day after the twenty-ninth of Sha'ban,
when the sighting of the crescent moon has not been confirmed.
In reality, this day is the thirtieth day of Sha'ban.

➋
Some people fast on the thirtieth of Sha'ban
thinking that perhaps Ramadan has begun and they are unaware of it.
If Ramadan has indeed started, then this fast will count as a Ramadan fast;
otherwise, it will be considered a voluntary (nafl) fast.
Fasting on such a day out of doubt is prohibited in the Shari'ah.

➌
Allah Ta'ala has determined the quantity and timings of obligatory acts of worship.
It is not correct to abolish the distinction between voluntary (nafl) and obligatory (fard) acts of worship.

➍
If a righteous deed is performed contrary to the Sunnah,
then it no longer remains a righteous deed.

➎
According to most researchers, this narration is authentic.
The reason why some of the Companions (radi Allahu anhum ajma'in) did not break their fast could be
that they were fasting as per their usual practice, which is permissible.