Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
muslim:1080c – Ibn Numayr from my father > ʿUbaydullāh Bihadhā al-Isnād

If (the sky) is cloudy for you, then calculate thirty days (for the month of Ramadan).  

مسلم:١٠٨٠c – وَحَدَّثَنَا ابْنُ نُمَيْرٍ حَدَّثَنَا أَبِي حَدَّثَنَا عُبَيْدُ اللَّهِ بِهَذَا الإِسْنَادِ

وَقَالَ فَإِنْ غُمَّ عَلَيْكُمْ فَاقْدِرُوا ثَلاَثِينَ نَحْوَ حَدِيثِ أَبِي أُسَامَةَ