حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ عَنْ عُبَيْدِ اللَّهِ حَدَّثَنِي نَافِعٌ عَنِ ابْنِ عُمَرَ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ الشَّهْرُ تِسْعٌ وَعِشْرُونَ هَكَذَا وَهَكَذَا فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدُرُوا لَهُ قَالَ وَكَانَ ابْنُ عُمَرَ إِذَا كَانَ لَيْلَةُ تِسْعٍ وَعِشْرِينَ وَكَانَ فِي السَّمَاءِ سَحَابٌ أَوْ قَتَرٌ أَصْبَحَ صَائِمًا
مسند امام احمد بن حنبل · Hadith 4611
It was narrated that Ibn 'Umar said: The Messenger of Allah (PBUH) said: The month is twenty-nine (days), like this and like this and like this. If it is cloudy, then work it out." If the sky was cloudy or it was raining on the night following the twenty-ninth (of Sha'baan), Ibn 'Umar would start to fast the next day. [2]
Translated by Dr. Muhammad Muhsin Khan · Reviewed by Islamic University of Madinah · Published March 2020