According to Shia Islamic belief and historical sources:

- The Prophet Muhammad received the first revelation of the Quran through the Archangel Gabriel when he was around 40 years old.

- The revelation occurred in the month of Ramadan, most likely the night of Power (Laylatul Qadr). This would correspond to the year 610 CE in the modern Gregorian calendar.

- The Prophet was in the cave of Hira, located on the mountain named Jabal an-Nur near Makkah. He had retreated there annually for prayer and reflection.

- The first words whispered to him by Gabriel were "IQRA' – Read!”. The command referred to both the act of reading and recitation of what would be the grand revelation - the Quran.

- The experience shook the Prophet greatly and he returned home, unable to comprehend fully what had transpired. His wife Khadija offered comfort and support.

- Over the following 23 years until his demise, the revelations continuing unfolding piece-by-piece through Gabriel to form the Islamic scripture.

So in short, the initial transmission of the divine messages from God that began the religion of Islam occurred in 610 CE when Prophet Muhammad was 40. This marked the start of his prophetic mission.

References

https://qr.ae/pyEXa5