At first what started of the Waḥī (Divine Inspiration) to the Messenger of Allah ﷺ was good visions during sleep and he never saw a vision except what came to him was the like of the crack of dawn (i.e. his visions came true). Thereafter, seclusion was made beloved to him. He used to go in seclusion in the cave of Ḥirāʾ where he used to practice monotheism, which was worship for several days until he longed to see his family and prepare for a similar trip. He would then return to Khadījah and would prepare for a similar trip.
Until ˹one day,˺ the Truth descended upon him while he was in the cave of Ḥirāʾ. The Angel came to him and asked him and said "Iqraʾ (Read!)". He said, "I do not know how to read!" He said, "The Angel grabbed me and overwhelmed me such that fatigue overcame me. He then let me go and and said 'Read.' I said, 'I do not know how to read!' Then he grabbed me a second time and overwhelmed me such that fatigue overcame me. He then let me go and said 'Read!' I said, 'I do not know how to read!' Then he grabbed me for the third time, overwhelmed me, and then let me go and said, 'Read in the name of your Lord: who created; He created man from ʿalaq (a thing suspended). Read! And your Lord is the Most Generous." (ʿAlaq Q 96:1-3)
Then Messenger of Allah ﷺ returned with the Inspiration and with his heart beating severely. Then he went to Khadija bint Khuwailid and said, "Cover me! Cover me!" They covered him till his fear was over and after that he told her everything that had happened and said, "I fear that something may happen to me." Khadija replied, "Never! By Allah, Allah will never disgrace you. You keep good relations with your kith and kin, help the poor and the destitute, serve your guests generously and assist the deserving calamity-afflicted ones."
Khadija then accompanied him to her cousin Waraqa bin Naufal bin Asad bin 'Abdul 'Uzza, who, during the pre-Islamic Period became a Christian and used to write the writing with Hebrew letters. He would write from the Gospel in Hebrew as much as Allah wished him to write. He was an old man and had lost his eyesight. Khadija said to Waraqa, "Listen to the story of your nephew, O my cousin!" Waraqa asked, "O my nephew! What have you seen?" Messenger of Allah ﷺ described whatever he had seen. Waraqa said, "This is the same one who keeps the secrets (angel Gabriel) whom Allah had sent to Moses. I wish I were young and could live up to the time when your people would turn you out." Messenger of Allah ﷺ asked, "Will they drive me out?" Waraqa replied in the affirmative and said, "Anyone (man) who came with something similar to what you have brought was treated with hostility; and if I should remain alive till the day when you will be turned out then I would support you strongly." But after a few days Waraqa died and the Divine Inspiration was also paused for a while. (Using translation from Bukhārī 3)
النَّبِيِّ ﷺ أَخْبَرَتْهُ قَالَتْ كَانَ أَوَّلَ مَا بُدِئَ بِهِ رَسُولُ اللهِ مِنَ الْوَحْيِ الرُّؤْيَا الصَّالِحَةُ فِي النَّوْمِ فَكَانَ لَا يَرَى رُؤْيَا إِلَّا جَاءَتْ مِثْلَ فَلَقِ الصُّبْحِ ثُمَّ حَبَّبَ اللهُ إِلَيْهِ الْخَلَاءَ فَكَانَ يَخْلُو بِغَارِ حِرَاءٍ فَيَتَحَنَّثُ فِيهِ وَهُوَ التَّعَبُّدُ اللَّيَالِيَ أُولَاتِ الْعَدَدِ قَبْلَ أَنْ يَرْجِعَ إِلَى أَهْلِهِ وَيَتَزَوَّدُ لِذَلِكَ ثُمَّ يَرْجِعُ إِلَى خَدِيجَةَ فَتُزَوِّدُهُ بِمِثْلِهَا حَتَّى فَجَأَهُ الْحَقُّ وَهُوَ فِي غَارِ حِرَاءٍ فَجَاءَهُ الْمَلَكُ فَقَالَ اقْرَأْ فَقَالَ مَا أَنَا بِقَارِئٍ قَالَ فَأَخَذَنِي فَغَطَّنِي حَتَّى بَلَغَ مِنِّي الْجَهْدُ ثُمَّ أَرْسَلَنِي فَقَالَ اقْرَأْ فَقُلْتُ مَا أَنَا بِقَارِئٍ فَأَخَذَنِي فَغَطَّنِي الثَّانِيَةَ حَتَّى بَلَغَ مِنِّي الْجَهْدُ ثُمَّ أَرْسَلَنِي فَقَالَ اقْرَأْ فَقُلْتُ مَا أَنَا بِقَارِئٍ فَأَخَذَنِي فَغَطَّنِي الثَّالِثَةَ حَتَّى بَلَغَ مِنِّي الْجَهْدُ ثُمَّ أَرْسَلَنِي فَقَالَ {اقْرَأْ بِاسْمِ رَبِّكَ الَّذِي خَلَقَ خَلَقَ الْإِنْسَانَ مِنْ عَلَقٍ اقْرَأْ وَرَبُّكَ الْأَكْرَمُ الَّذِي عَلَّمَ بِالْقَلَمِ عَلَّمَ الْإِنْسَانَ مَا لَمْ يَعْلَمْ} [العلق 2] فَرَجَعَ بِهَا رَسُولُ اللهِ ﷺ تَرْجُفُ بَوَادِرُهُ حَتَّى دَخَلَ عَلَى خَدِيجَةَ ؓ فَقَالَ زَمِّلُونِي زَمِّلُونِي فَزَمَّلُوهُ حَتَّى ذَهَبَ عَنْهُ الرَّوْعُ ثُمَّ قَالَ لِخَدِيجَةَ أَيْ خَدِيجَةُ مَا لِي وَأَخْبَرَهَا الْخَبَرَ قَالَ لقَدْ خَشِيتُ عَلَى نَفْسِي قَالَتْ لَهُ خَدِيجَةُ كَلَّا أَبْشِرْ فَوَاللهِ لَا يُخْزِيكَ اللهُ أَبَدًا وَاللهِ إِنَّكَ لِتَصِلُ الرَّحِمَ وَتَصْدُقُ الْحَدِيثَ وَتَحْمِلُ الْكَلَّ وَتُكْسِبُ الْمَعْدُومَ وَتَقْرِي الضَّيْفَ وَتُعِينُ عَلَى نَوَائِبِ الْحَقِّ فَانْطَلَقَتْ بِهِ خَدِيجَةُ ؓ حَتَّى أَتَتْ بِهِ وَرَقَةَ بْنَ نَوْفَلِ بْنِ أَسَدِ بْنِ عَبْدِ الْعُزَّى بْنِ قُصَيٍّ وَهُوَ ابْنُ عَمِّ خَدِيجَةَ ابْنُ أَخِي أَبِيهَا وَكَانَ امْرَأً تَنَصَّرَ فِي الْجَاهِلِيَّةِ يَكْتُبُ الْكِتَابَ الْعَرَبِيَّ وَيَكْتُبُ مِنَ الْإِنْجِيلِ بِالْعَرَبِيَّةِ مَا شَاءَ اللهُ أَنْ يَكْتُبَ وَكَانَ شَيْخًا كَبِيرًا قَدْ عَمِيَ فَقَالَتْ لَهُ خَدِيجَةُ أَيْ عَمِّ اسْمَعْ مِنِ ابْنِ أَخِيكَ قَالَ وَرَقَةُ بْنُ نَوْفَلٍ ابْنَ أَخِي مَاذَا تَرَى؟ فَأَخْبَرَهُ رَسُولُ اللهِ ﷺ خَبَرَ مَا رَأَى فَقَالَ لَهُ وَرَقَةُ هَذَا النَّامُوسُ الَّذِي نَزَّلَ اللهُ عَلَى مُوسَى يَا لَيْتَنِي فِيهَا جَذَعًا يَا لَيْتَنِي أَكُونُ حَيًّا حِينَ يُخْرِجُكَ قَوْمُكَ قَالَ رَسُولُ اللهِ ﷺ أَوَمُخْرِجِيَّ هُمْ؟ قَالَ وَرَقَةُ نَعَمْ لَمْ يَأْتِ رَجُلٌ قَطُّ بِمَا جِئْتَ بِهِ إِلَّا عُودِيَ وَإِنْ يُدْرِكْنِي يَوْمُكَ أَنْصُرْكَ نَصْرًا مُؤَزَّرًا