Sahih (Sahih Muslim)
وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، بِهَذَا الإِسْنَادِ وَقَالَ " فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا ثَلاَثِينَ " . نَحْوَ حَدِيثِ أَبِي أُسَامَةَ .
This hadith is narrated on the authority of 'Ubaidullah with the same chain of transmitters, and he said:If (the sky) is cloudy for you, then calculate thirty days (for the month of Ramadan)