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).

وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، بِهَذَا الإِسْنَادِ وَقَالَ ‏

‏ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا ثَلاَثِينَ ‏

‏ ‏.‏ نَحْوَ حَدِيثِ أَبِي أُسَامَةَ ‏.‏