صحيح البخاري Sahih Bukhari كتاب الصوم The Book of As-Saum (The Fasting).
بَابُ وُجُوبِ صَوْمِ رَمَضَانَ:

(1) Chapter. Fasting is obligatory in (the month of) Ramadan.

3 hadith
Hadith 1891
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ ، عَنْ أَبِي سُهَيْلٍ ، عَنْ أَبِيهِ ، عَنْ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ ، أَنَّ أَعْرَابِيًّا جَاءَ إِلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ثَائِرَ الرَّأْسِ ، فَقَالَ : " يَا رَسُولَ اللَّهِ ، أَخْبِرْنِي مَاذَا فَرَضَ اللَّهُ عَلَيَّ مِنَ الصَّلَاةِ ؟ فَقَالَ : الصَّلَوَاتِ الْخَمْسَ ، إِلَّا أَنْ تَطَّوَّعَ شَيْئًا ، فَقَالَ : أَخْبِرْنِي مَا فَرَضَ اللَّهُ عَلَيَّ مِنَ الصِّيَامِ ؟ فَقَالَ : شَهْرَ رَمَضَانَ ، إِلَّا أَنْ تَطَّوَّعَ شَيْئًا ، فَقَالَ : أَخْبِرْنِي بِمَا فَرَضَ اللَّهُ عَلَيَّ مِنَ الزَّكَاةِ ؟ فَقَالَ : فَأَخْبَرَهُ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ شَرَائِعَ الْإِسْلَامِ ، قَالَ : وَالَّذِي أَكْرَمَكَ لَا أَتَطَوَّعُ شَيْئًا وَلَا أَنْقُصُ مِمَّا فَرَضَ اللَّهُ عَلَيَّ شَيْئًا ، فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : أَفْلَحَ إِنْ صَدَقَ ، أَوْ دَخَلَ الْجَنَّةَ إِنْ صَدَقَ " .
Narrated Talha bin 'Ubaidullah: A bedouin with unkempt hair came to Allah's Apostle and said, "O Allah's Apostle! Inform me what Allah has made compulsory for me as regards the prayers." He replied: "You have to offer perfectly the five compulsory prayers in a day and night (24 hours), unless you want to pray Nawafil." The bedouin further asked, "Inform me what Allah has made compulsory for me as regards fasting." He replied, "You have to fast during the whole month of Ramadan, unless you want to fast more as Nawafil." The bedouin further asked, "Tell me how much Zakat Allah has enjoined on me." Thus, Allah's Apostle informed him about all the rules (i.e. fundamentals) of Islam. The bedouin then said, "By Him Who has honored you, I will neither perform any Nawafil nor will I decrease what Allah has enjoined on me. Allah's Apostle said, "If he is saying the truth, he will succeed (or he will be granted Paradise).
Hadith Reference Sahih Bukhari / The Book of As-Saum (The Fasting). / Hadith: 1891
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة
Hadith 1892
حَدَّثَنَا مُسَدَّدٌ ، حَدَّثَنَا إِسْمَاعِيلُ ، عَنْ أَيُّوبَ ، عَنْ نَافِعٍ ، عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُ ، قَالَ : " صَامَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَاشُورَاءَ ، وَأَمَرَ بِصِيَامِهِ ، فَلَمَّا فُرِضَ رَمَضَانُ تُرِكَ " ، وَكَانَ عَبْدُ اللَّهِ لَا يَصُومُهُ ، إِلَّا أَنْ يُوَافِقَ صَوْمَهُ .
Narrated Ibn `Umar: The Prophet observed the fast on the 10th of Muharram ('Ashura), and ordered (Muslims) to fast on that day, but when the fasting of the month of Ramadan was prescribed, the fasting of the 'Ashura' was abandoned. `Abdullah did not use to fast on that day unless it coincided with his routine fasting by chance.
Hadith Reference Sahih Bukhari / The Book of As-Saum (The Fasting). / Hadith: 1892
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة
Hadith 1893
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا اللَّيْثُ ، عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ ، أَنَّ عِرَاكَ بْنَ مَالِكٍ حَدَّثَهُ ، أَنَّ عُرْوَةَ أَخْبَرَهُ ، عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا ، " أَنَّ قُرَيْشًا كَانَتْ تَصُومُ يَوْمَ عَاشُورَاءَ فِي الْجَاهِلِيَّةِ ، ثُمَّ أَمَرَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِصِيَامِهِ حَتَّى فُرِضَ رَمَضَانُ ، وَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : مَنْ شَاءَ فَلْيَصُمْهُ ، وَمَنْ شَاءَ أَفْطَرَ " .
Narrated `Aisha: (The tribe of) Quraish used to fast on the day of Ashura' in the Pre-Islamic period, and then Allah's Apostle ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed; whereupon the Prophet said, "He who wants to fast (on 'Ashura') may fast, and he who does not want to fast may not fast."
Hadith Reference Sahih Bukhari / The Book of As-Saum (The Fasting). / Hadith: 1893
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة