Messenger of Allah 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."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ الشَّهْرُ تِسْعٌ وَعِشْرُونَ لَيْلَةً فَلاَ تَصُومُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ  

" I am surprised at those who anticipate the moth, when the Messenger of Allah said: 'When you see the new crescent then fast, and when you see it, then stop fasting, and if it is obscured from you (too cloudy), then complete thirty days.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا رَأَيْتُمُ الْهِلاَلَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ  

Yahya related to me from Malik from Thawr ibn Zayd ad-Dili from Abdullah ibn Abbas that the Messenger of Allah ﷺ once mentioned Ramadan and said, "Do not start the fast or break it until you see the new moon. If the new moon is obscured from you, then complete a full thirty days."  

أَنَّ رَسُولَ اللَّهِ ﷺ ذَكَرَ رَمَضَانَ فَقَالَ لاَ تَصُومُوا حَتَّى تَرَوُا الْهِلاَلَ وَلاَ تُفْطِرُوا حَتَّى تَرَوْهُ فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاَثِينَ  

"The Messenger of Allah said; 'Fast when you see it and stop fasting when you see it, and if it is obscured from you (too cloudy), then complete thirty (days)."' (Using translation from Nasāʾī 2124)  

قَالَ رَسُولُ اللهِ ﷺ «صُومُوا الْهِلَالَ لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلَاثِينَ»  

" I am surprised at those who anticipate the moth, when the Messenger of Allah said: 'When you see the new crescent then fast, and when you see it, then stop fasting, and if it is obscured from you (too cloudy), then complete thirty days. (Using translation from Nasāʾī 2125)  

قَالَ رَسُولُ اللهِ ﷺ «إِذَا رَأَيْتُمُ الْهِلَالَ فَصُومُوا وَإِذَا رَأَيْتُمُوهُ فَأَفْطِرُوا فَإِنْ غُمَّ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلَاثِينَ»  

"الْهلالُ (صُومُوا) لرؤْيَتهِ، وأَفْطِرُوا لرُؤْيَتهِ، فَإِنْ غُمَّ عَلَيكُمْ فَأَكْمِلُوا الْعِدَّةَ ثَلاثينَ".  

"الشَّهْرُ تسعٌ وعِشْرُونَ فَلَا تَصُوموا حَتَّى تَرَوْهُ ولا تُفْطِرُوا حَتَّى تَرَوْهُ، فإِنْ غُمَّ عَلَيكم فأَكْمِلُوا العِدَّةَ ثَلاثينَ".