Praise be to Allah.

The scholars are unanimously agreed that fasting the month of Ramadaan was enjoined in Sha’baan 2 AH, and that the Prophet (peace and blessings of Allaah be upon him) fasted nine Ramadaans, because he (peace and blessings of Allaah be upon him) died in Rabee’ al-Awwal 11 AH. 

It says in al-Insaaf: 

Fasting Ramadaan was enjoined in 2 AH according to scholarly consensus. The Messenger of Allaah (peace and blessings of Allaah be upon him) fasted nine Ramadaans according to scholarly consensus. 

Al-Nawawi said in al-Majmoo’: 

The Messenger of Allaah (peace and blessings of Allaah be upon him) fasted Ramadaan for nine years, because it was enjoined in Sha’baan 2 AH, and the Prophet (peace and blessings of Allaah be upon him) died in Rabee’ al-Awwal 11 AH. 

See also Majmoo’ Fataawa by Shaykh al-Islam Ibn Taymiyah, 2/20.