Rasul Allah (sal Allahu alaihi wa sallam) said: “Islam is built upon five pillars: testifying that there is no God except Allah and that Muhammad is His Messenger, the establishment of the prayer, the giving of Zakah, the fast of Ramadan and the pilgrimage (Hajj) to Makkah.” [Sahih Bukhari]
A man came to the Prophet (sal Allahu alaihi wa sallam) and said: “O Messenger of Allah, tell me what Allah requires of me as regards fasting.” He answered: “The month of Ramadan.” The man asked: “Is there any other (fast)?” The Prophet (sal Allahu alaihi wa sallam) answered: “No, unless you do so voluntarily.” [Sahih Bukhari]
The edifice of Islam is built on five pillars. The fasts of Ramadhan are one of the pillars that support the structure of Islam. The whole Muslim nation agrees that the fast of Ramadan is obligatory. It is one of the pillars of Islam, and if one disputes this, he cannot be called a Muslim.