The correct answer to this open question is the following.
On that first revelation, Angel Gabriel told Muhammad that Muslims should only worship and obey Allah. Profet Muhannad's first revelation instructed him to teach Muslims to read the Quran.
According to Islam, this revelation occurred in 610 AD. This took place in the cave called Hira, un the Jabal al-Nour mountain. Prophet Muhammed was on meditation when Angel Gabriel came in and told Muhammad to "Read/Proclaim the name of your Lord who created man from a clinging substance..."
