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). Translation Not Available

Book Ref: Sahih Muslim Book 13 Hadith 2365
Web Ref:  Sahih Muslim Book 6 Hadith 1080 c