Fasting is obligatory in (the month of) Ramadan

Narrated Talha bin 'Ubaid-Ullah:

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 ﷺ said, "If he is saying the truth, he will succeed (or he will be granted Paradise)."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، عَنْ أَبِي سُهَيْلٍ، عَنْ أَبِيهِ، عَنْ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ، أَنَّ أَعْرَابِيًّا، جَاءَ إِلَى رَسُولِ اللَّهِ ﷺ ثَائِرَ الرَّأْسِ فَقَالَ يَا رَسُولَ اللَّهِ أَخْبِرْنِي مَاذَا فَرَضَ اللَّهُ عَلَىَّ مِنَ الصَّلاَةِ فَقَالَ " الصَّلَوَاتِ الْخَمْسَ، إِلاَّ أَنْ تَطَّوَّعَ شَيْئًا ". فَقَالَ أَخْبِرْنِي مَا فَرَضَ اللَّهُ عَلَىَّ مِنَ الصِّيَامِ فَقَالَ " شَهْرَ رَمَضَانَ، إِلاَّ أَنْ تَطَّوَّعَ شَيْئًا ". فَقَالَ أَخْبِرْنِي بِمَا فَرَضَ اللَّهُ عَلَىَّ مِنَ الزَّكَاةِ فَقَالَ فَأَخْبَرَهُ رَسُولُ اللَّهِ ﷺ شَرَائِعَ الإِسْلاَمِ. قَالَ وَالَّذِي أَكْرَمَكَ لاَ أَتَطَوَّعُ شَيْئًا، وَلاَ أَنْقُصُ مِمَّا فَرَضَ اللَّهُ عَلَىَّ شَيْئًا. فَقَالَ رَسُولُ اللَّهِ ﷺ " أَفْلَحَ إِنْ صَدَقَ، أَوْ دَخَلَ الْجَنَّةَ إِنْ صَدَقَ ".

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.

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا إِسْمَاعِيلُ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ صَامَ النَّبِيُّ ﷺ عَاشُورَاءَ، وَأَمَرَ بِصِيَامِهِ. فَلَمَّا فُرِضَ رَمَضَانُ تُرِكَ. وَكَانَ عَبْدُ اللَّهِ لاَ يَصُومُهُ، إِلاَّ أَنْ يُوَافِقَ صَوْمَهُ.