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 sisters! 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 son Sayyiduna Isma’eel عَـلَيْـهِ الـسَّـلَام. Allah عَزَّوَجَلَّ loved this act of Sayyidatuna Haajirah رَضِیَ اللهُ تَعَالٰی عَنْهَا, therefore to make this blessed Sunnah of Sayyidatuna Haajirah رَضِیَ اللهُ تَعَالٰی عَنْهَا remain alive, Allah عَزَّوَجَلَّ made the Sa’ee of Safa and Marwah Wajib for Hajjis and Umrah-performing people. In the same way, our Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم spent some days of blessed Ramadan in the cave of Hira. During this period of time, he صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم would avoid eating during the day and get busy doing Zikr of Allah عَزَّوَجَلَّ in the night. Hence in order to remind us of these glorious days, Allah عَزَّوَجَلَّ has made Siyam obligatory so that the Sunnah of His Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم continues to exist. (Faizan-e-Sunnat, pp. 935)

How strong is faith of Sawm-observing person!

Dear Islamic sisters! It is very hot, the throat and lips are getting dry due to thirst, water is available, but a Sawm-observing person does not even look at it; food is available, but she does not extend even her hand to it despite being very hungry. Just imagine, how strong the faith of this person is in Allah عَزَّوَجَلَّ! It is because she knows that her actions can be hidden from the entire world, but they cannot be concealed from Allah عَزَّوَجَلَّ. This belief of her in Allah عَزَّوَجَلَّ is the practical result of a perfect Sawm, because other acts of worship are performed by any apparent movements, but Sawm is related to internal [form of worship] whose state no one knows except Allah عَزَّوَجَلَّ. If she (Sawm observing person) eats and drinks something secretly, people will still feel that she is observing Sawm, however, she is protecting herself from eating and drinking only due to Divine fear. That is why the Sawm-observing people will be blessed with abundant gifts and rewards. Let’s listen to three sayings of the Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم about the virtues of Sawm and rejoice: