Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah ﷺ once mentioned Ramadan and said, "Do not begin the fast until you see the new moon, and do not break the fast (at the end of Ramadan) until you see it. If the new moon is obscured from you, then work out (when it should be)."
أَنَّ رَسُولَ اللَّهِ ﷺ ذَكَرَ رَمَضَانَ فَقَالَ لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ