Prophet Muhammad started to receive revelation again in 1 BH

The Prophet (peace be upon him) kept waiting for revelation until it eventually came again. The Prophet (peace be upon him) told us about when revelation started to come again, saying «When I was walking, I heard a voice from the sky, so I raised my head. I saw the angel who had come to me in Hira cave sitting on a chair between the heavens and earth. I started to feel scared and therefore I returned to my house and then said, ‘Cover me, cover me.’ As a result, they covered me and then Allah (Glorified is He) revealed, {O you wrapped up [in a mantle]! Arise and deliver the warning. And your Lord do magnify. And your garments keep free from impurities. And all abomination shun} [Surah Al-Muddathur: 1-5]. After that, revelation came one after another» [related by Al-Bukhari and Muslim].

In the first ayahs of Surah Al-Muddathur, Allah (Glorified is He) ordered the Prophet (peace be upon him) to warn the people and call them to worship Allah alone, taking no partners with Him. Therefore, this was a dividing line between two major stages in the Prophet’s life: the stage before being sent to the people, which entailed most of his life and during which Allah (Glorified is He) did not give him any special responsibilities, and the stage after being sent to preach to the people which was the most dangerous and difficult stage in the Prophet’s life as it was the stage that changed the history of mankind. This stage entailed divine orders; Allah (Glorified is He) ordered him to leave sleepiness and to roll up his sleeves to exert effort to change the course of mankind, saving them from the ruin and evil which they were immersed in and leading them to the path of success which results in happiness in this life and the great success in the next life [adapted from The Blessed Garden].

On this day, the journey of prophethood, proclamation and warning mankind started. This was a hard, long and difficult journey. On that day, the Prophet (peace be upon him) started to call others to Allah (Glorified is He) and spread the Message to the world, carrying a heavy burden on his shoulders which was as heavy as a mountain: the burden of a great trust which he carried to all the earth and a great Message which he was ordered to convey to all the world.