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.
ahmad:14670Ḥasan > Ibn Lahīʿah > Abū al-Zubayr > Saʾalt Jābir Hal

[Machine] I heard the Messenger of Allah ﷺ say, "Do not fast until you see the crescent moon, and if it is hidden from you, then complete thirty (days)." Jabir reported that the Messenger of Allah ﷺ once abandoned fasting for a month, so it became twenty-nine, and he said, "The month is only twenty-nine days."  

أحمد:١٤٦٧٠حَدَّثَنَا حَسَنٌ حَدَّثَنَا ابْنُ لَهِيعَةَ حَدَّثَنَا أَبُو الزُّبَيْرِ قَالَ سَأَلْتُ جَابِرًا هَلْ

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ لَا تَصُومُوا حَتَّى تَرَوْا الْهِلَالَ فَإِنْ خَفِيَ عَلَيْكُمْ فَأَتِمُّوا ثَلَاثِينَ ؟ وَقَالَ جَابِرٌ هَجَرَ رَسُولُ اللهِ ﷺ نِسَاءَهُ شَهْرًا فَنَزَلَ لِتِسْعٍ وَعِشْرِينَ وَقَالَ إِنَّمَا الشَّهْرُ تِسْعٌ وَعِشْرُونَ