NASAI-KUBRA 2442:
from Ibn ʿUmar —
Messenger of Allah 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). (Using translation from Bukhārī 1906)
الكبرى للنسائي ٢٤٤٢:
عن ابن عمر —
أَنَّ رَسُولَ اللهِ ﷺ ذَكَرَ رَمَضَانَ فَقَالَ «لَا تَصُومُوا حَتَّى تَرَوُا الْهِلَالَ وَلَا تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ»
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