The fast of Ramadhaan was made fardh (obligatory) upon the Ummah in the 2nd year after hijrah. ^[1] Allah Ta‘ala says in the Quraan Majeed:

يَاأَيُّهَا الَّذِيْنَ آمَنُوْا كُتِبَ عَلَيْكُمُ الصِّيَامُ كَمَا كُتِبَ عَلَى الَّذِيْنَ مِنْ قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُوْنَ

 “Fasting has been ordained upon you as it was ordained upon those before you so that you may attain taqwa.” ^[2]