Sunan an-Nasa'i 2129
سنن النسائي#2129SahihIbn 'Abbas narrated that the Messenger of Allah said:"Fast when you see it, and stop fasting when you see it, and if clouds prevent you from seeing it, then complete the number, and do not fast ahead of Ramadan
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 22:40