When was Ramadhan made obligatory?

Imaam Al-Nawawee (رحمه الله) said in his book 'Sharh-ul-Almuhath', vol:6/250 :

" The Messenger of Allaah fasted for nine years ( Ramadan) because Ramadhan was made obligatory in the month of Sha'baan two years after the Hijrah."

Also Sheikh Abdul-Allaah Al-B said in his book ‘Toodeeh ul Al-Ahkaam’ (vol:3/129)."

"..And it was made obligatory the fasting of Ramadhan in Sha'baan two years after Hijrah so the Messenger of Allaah fasted nine Ramadan's all together."

-- Taken from the book of Sheikh Muhammad Al Hizaam Itihaf-ul-Anaam
Translated by Abu Rajab Ahmed As-Sheikhee