Narrated Aishah, Ummul Muminin: The Messenger of Allah ﷺ used to count the days in Sha'ban in a manner he did not count any other month; then he fasted when he sighted the new moon of Ramadan; but if the weather was cloudy he counted thirty days and then fasted.
Hadith Referenceسنن ابي داود / كتاب الصيام / 2325
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:إسناده صحيح, مشكوة المصابيح (1980), صححه ابن خزيمة (1910 وسنده صحيح)
Narrated Hudhayfah: The Prophet ﷺ said: Do not fast (for Ramadan) before the coming of the month until you sight the moon or complete the number (of thirty days); then fast until you sight the moon or complete the number (of thirty days).
Hadith Referenceسنن ابي داود / كتاب الصيام / 2326
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:إسناده صحيح, أخرجه النسائي (2128 وسنده صحيح) وصححه ابن خزيمة (1911 وسنده صحيح)