Sunan Ibn Majah 1654
سنن ابن ماجه#1654SahihIt was narrated from Ibn ‘Umar that the Messenger of Allah (ﷺ) 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).” Ibn ‘Umar used to fast one day before the new crescent was seen
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih - Bukhari And Muslim
In-book reference: 7:17