" I heard the Messenger of Allah (may Allah's peace and blessings be upon him) saying: ""Observe the fast when you see it (the new moon of Ramadan), and break the fast when you see it (the new moon of Shawwāl), but if the sky is overcast and you cannot see it, then act on estimation (by counting Sha‘bān as 30 days)."" [Al-Bukhari]"

"عن عبد الله بن عمر -رضي الله عنهما- قال: سمعت رسول الله -صلى الله عليه وسلم- يقول: «إذا رَأَيْتمُوه فَصُومُوا، وإذا رَأَيْتُمُوه فَأفْطِروُا، فإن غُمَّ عليكم فَاقْدُرُوا له». [صحيح.] - [متفق عليه.]"