Sahih al-Bukhari 1900
Sahih al-Bukhari · Book 30: Fasting · Should it be said "Ramadan" or "the month of Ramadan"? And whoever thinks that both are permissible
Narrated Ibn `Umar:
حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ ، قَالَ حَدَّثَنِي اللَّيْثُ ، عَنْ عُقَيْلٍ ، عَنِ ابْنِ شِهَابٍ ، قَالَ أَخْبَرَنِي سَالِمٌ ، أَنَّ ابْنَ عُمَرَ ـ رضى الله عنهما ـ قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " إِذَا رَأَيْتُمُوهُ فَصُومُوا، وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا، فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ " . وَقَالَ غَيْرُهُ عَنِ اللَّيْثِ حَدَّثَنِي عُقَيْلٌ وَيُونُسُ لِهِلاَلِ رَمَضَانَ .
I heard Allah's Messenger (ﷺ) saying, "When you see the crescent (of the month of Ramadan), start fasting, and when you see the crescent (of the month of Shawwal), stop fasting; and if the sky is overcast (and you can't see it) then regard the month of Ramadan as of 30 days."