Advent (Be ‘sat) of the Noble Messenger (s.a.w.a.s.) was the occasion when the Omnipotent Allah (s.w.t.) sent the Arch-Angel Gabriel to Noble Messenger (s.a.w.a.s.) in the cave of Hira near Makkah. Gabriel brought the first 5 Ayahs of Surah al-Alaq which says, “(Muhammad), read in the name of your Lord Who created (all things). He created man from a clot of blood. Recite! Your Lord is most Honorable One, Who by the pen taught the human being: He taught the human being what he did not know.” It was the 27th Rajab 40th year of Elephant (610 C.E.). This was the start of the revelation of the Glorious Qur’an which continued for 23 years. Hence, the Divine Mission began to proclaim the Unity of Allah and the equality of mankind.
The Noble Messenger (s.a.w.a.s.)’s prophet hood was planned by Allah (s.w.t.) long before his birth in the world and a covenant was obtained from all prophets as stated in Surah Al-Imran Ayah 81, “When Allah made a covenant with the Prophets, saying, “When I give you the Book and wisdom and a Messenger comes to you who will testify to the guidance which you have received from Me, you must believe in him and help him,“ He then asked them, “Do you affirm and accept my covenant?” They replied, “Yes, we affirm it.” The Lord said, “Then bear witness to this and I shall bear witness with you.”
His (s.a.w.a.s.) wife Sayyida Khadija (s.a.) was the first to testify his truth followed by his cousin Ali (a.s.).
There is some misconception in certain quarters that he (s.a.w.a.s.) was awarded prophet hood at the age of 40 years. This is incorrect. According to Ibn Abbas, the Noble Messenger (s.a.w.a.s.) himself said, “I was a prophet even when Adam was between clay and water.” (al-Tabarani, al-Mujjam al-Kabir, al-Khasa’is al-Kubra v. 1 page 4). At another place he said, “Allah created my light first” (Jabir ibn Abdullah Ansari in al-Qastalani volume 1 page 5,9,10). Allah said to the light of Noble Messenger (s.a.w.a.s.), “It is because of you that I am going to create the earth and the skies, lay down reward and punishment, and bring into being the Garden and the Fire. Then the tradition goes on to speak about Ahlal Bayt” (al-Masudi in Maruju’dh-dhahb, also in famous Hadees-e-Kissa’)
Hence, the Noble Messenger (s.a.w.a.s.) was a born prophet. He was among the people of Makkah for 40 years and by his matchless character, the infidels gave him the title of “Truthful” and “Trust-worthy”. He spent most of his time in Mount Hira in meditation and solitude. No one could go there except Sayyida Khadija (s.a.) and the young Imam Ali (a.s.) the Commander of the Faithful.
Surah al-Hijr Ayah 94, “Preach what have been commanded to and stay away from the pagans.” Surah al-Muddaththir Ayah 1 to 3, “O Cloaked one, stand up, deliver your warning, proclaim the greatness of your Lord.”
Initially, he invited people to Islam secretly. After three years, he was commanded by Allah to openly spread the word of Unity of Allah in Surah al-Shu’ra Ayah 214, “Warn your close relatives.” The Noble Messenger (s.a.w.a.s.) told the Commander of the Faithful Imam Ali (a.s.) about the Divine Command and asked him to arrange for the first feast in the Islamic history called “Dawat-e-Zul’Ashirah” which was held in the house of Hazrat Abu Talib (a.s.), the Guardian and the Protector of the Noble Messenger (s.a.w.a.s.)
Sources: The Glorious Qur’an, imamreza.net, al-muntazar, Life of Muhammad by Sayyid Saeed Akhtar Rizvi