It was narrated from Ibn “Umar that the Messenger of Allah (saws) said:
“When you see the new crescent 🌙, fast, and when you see it, stop fasting. If it is cloudy then calculate it (as thirty days).”
Abdullah Ibn”Umar used to fast one day before the new crescent 🌙 was seen.
Sunan ibn majah. Hadith 1654
: Book 7, Hadith 17
a. A lunar month begins with the appearance of the new crescent. The night on which the moon appears is counted with the following day.
b. Estimating when it is cloudy, means completing thrty days of fast. As another narration’s version is ‘if it is cloudy then complete the number as thirty days of Ramadan
c. Ibn ‘Umar fasted a day before Ramdan
possibly it was coincided with his routine fasting that accidentally happened on this day
It was narrated from Abu Hurairah that the Messenger of Allah (saws) said:
“When you see the new crescent 🌙 then fast, and when you see it then stop fasting. If it is cloudy then fast thirty days (complete 30 days of Ramadan)
Sunan ibn majah. Hadith 1655
Book 7, Hadith 18
It was narrated that Abu Hurairah said:
“(The months in which) We fasted twenty-nine days at the time of the Allah’s massanger (saws), were more than (the months in which) we fasted thirty days (during Ramadan).
Sunan ibn majah. Hadith 1658
Book 7, Hadith 1727
* it clear that a month is also of twenty-nine days; it is not necessarily thirty days. There is no decrease in reward for the fast of a month in case of it being twenty-nine days.