(7) باب مَنْ قَالَ فَإِنْ غُمَّ عَلَيْكُمْ فَصُومُوا ثَلاَثِينَ
Narrated Abdullah ibn Abbas:
The Prophet (ﷺ) said: Do not fast one day or two days just before Ramadan except in the case of a man who has been in the habit or observing a fast (on that day); and do not fast until you sight it (the moon). Then fast until you sight it. If a cloud appears on that day (i.e. 29th of Ramadan) then complete the number thirty (days) and then end the fasting: a month consists of twenty-nine days. (DE)
Reference (DE): Sunan Abi Dawud 2327 (DE)
In-book reference (DE): Book 14, Hadith 15 (DE)
English translation (DE): Book 13, Hadith 2320 (DE)