Allah's Messenger (ﷺ) said, "The month (can be) 29 nights (i.e. days), and do not fast till you see the moon, and if the sky is overcast, then complete Sha'ban as thirty days."
Sahih al-Bukhari Book 30, Hadith 17
← Previous Hadith Allah's Messenger (ﷺ) mentioned Ramadan and said, "Do not fast unless you see the crescent (of Ramadan), and do not give up fasting till you see the crescent (of Shawwal), but if the sky is overcast (if you cannot see it), then act on estimation (i.e. count Sha'ban as 30 days).

Sahih al-Bukhari Book 30, Hadith 16 Next Hadith → The Prophet (ﷺ) said, "The month is like this and this," (at the same time he showed the fingers of both his hands thrice) and left out one thumb on the third time.

Sahih al-Bukhari Book 30, Hadith 18