Surah Al-Baqarah, part 2, Allah عَزَّوَجَلَّ has stated about the revelation of the Holy Quran and the month of Ramadan:

شَهْرُ رَمَضَانَ الَّذِیْۤ اُنْزِلَ فِیْهِ الْقُرْاٰنُ هُدًى لِّلنَّاسِ وَ بَیِّنٰتٍ مِّنَ الْهُدٰى وَ الْفُرْقَانِۚ-فَمَنْ شَهِدَ مِنْكُمُ الشَّهْرَ فَلْیَصُمْهُؕ-وَ مَنْ كَانَ مَرِیْضًا اَوْ عَلٰى سَفَرٍ فَعِدَّةٌ مِّنْ اَیَّامٍ اُخَرَؕ-یُرِیْدُ اللّٰهُ بِكُمُ الْیُسْرَ وَ لَا یُرِیْدُ بِكُمُ الْعُسْرَ٘-وَ لِتُكْمِلُوا الْعِدَّةَ وَ لِتُكَبِّرُوا اللّٰهَ عَلٰى مَا هَدٰىكُمْ وَ لَعَلَّكُمْ تَشْكُرُوْنَ(۱۸۵)

Translation from Kanz-ul-Iman: The month of Ramadan in which was sent down the Quran - the guidance for mankind, the direction and the clear criterion (to judge between right and wrong). So whoever among you finds this month, must fast for the (whole) month; and whoever is sick or on a journey, may fast the same number in other days. Allah desires ease for you and does not desire hardship for you so that you complete the count (of fasts), and glorify Allah as He has guided you, and so that you may be grateful. (Part 2, Surah Al-Baqarah, Ayah 185)

Dear Islamic brothers! In this blessed Ayah, Allah عَزَّوَجَلَّ has also commanded us to observe Siyam in the blessed month of Ramadan. Remember! After admitting the oneness of Allah عَزَّوَجَلَّ and Risalat and having faith in all the essentials of Islam the way Salah has been made obligatory for every Muslim, similarly, the Siyam of blessed Ramadan have also been made obligatory for every Muslim (man and woman) sane and the one who has reached puberty. It is stated in Durr-e-Mukhtar: Siyam were made obligatory on the 10 Sha’ban-ul-Mu’azzam 2 Hijri. (Durr-e-Mukhtar ma’ Rad-dul-Muhtar, vol. 3, pp. 330)

Reason why Sawm was made obligatory

Dear Islamic brothers! Most of the acts in Islam have been fixed to remind us of some faith-refreshing events. For example, the Sa’ee of Hajis between Safa and Marwah reminds us of Sayyidatuna Haajirah رَضِیَ اللهُ تَعَالٰی عَنْهَا. She walked and ran between both these hills seven times in search of water for her Beloved