Khadijah bint Khuwaylid (may Allah be pleased with her): Khadijah taking the Prophet (peace and blessings be upon him) to Waraqah, and his affirmation

It is narrated from Afif bin Amr; he said: I was a merchant, and in the period of Jahiliyyah I was a friend of Abbas bin Abd al-Muttalib. I came for trade and stayed with Abbas bin Abd al-Muttalib at Mina. (There I saw that) a man came, and when the sun had passed its zenith he stood up and began to pray. Then a woman came, and she too stood and began to pray. Then a boy who was close to puberty came, and he too stood and began to pray. I asked Abbas: "Who is this?" He said: "This is Muhammad bin Abdullah bin Abd al-Muttalib, my nephew. He claims that he is a Prophet, and no one has followed him in this matter except this woman and this boy. This woman is Khadijah bint Khuwaylid, his wife, and this boy is his cousin, Ali bin Abi Talib." Afif bin Amr (after he had accepted Islam and his Islam had become excellent) used to say: "I wish I had accepted Islam that day; then I would have been a quarter of Islam (the fourth Muslim)."
This hadith has a sahih (authentic) chain of narration, and the two Shaykhs did not record it.

It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the revelation to the Messenger of Allah (peace and blessings be upon him) began with true dreams in sleep. Whatever dream he (peace and blessings be upon him) saw would come to pass as clearly as the light of dawn. Then seclusion was made beloved to him (peace and blessings be upon him), and he (peace and blessings be upon him) would go to the cave of Hira and engage in "tahannuth", that is, worship, until the truth came to him (peace and blessings be upon him) while he (peace and blessings be upon him) was in the cave of Hira. There the angel came to him (peace and blessings be upon him) and said: "Read." He (peace and blessings be upon him) said: "I am not one who can read." He (peace and blessings be upon him) said: "Then the angel seized me and squeezed me hard until my strength began to give way. Then he released me and said: 'Read in the name of your Lord who created. He created man from a clinging clot. Read, and your Lord is the Most Generous, who taught by the pen, taught man what he did not know' [Surah al-Alaq: 1-5]." The narrator said: Then the Messenger of Allah (peace and blessings be upon him) returned with these verses, his (peace and blessings be upon him) shoulders trembling, until he (peace and blessings be upon him) reached Sayyidah Khadijah (may Allah be pleased with her) and said: "Wrap me up, wrap me up." So they wrapped him (peace and blessings be upon him) up until his (peace and blessings be upon him) fear left him. Then he (peace and blessings be upon him) said: "O Khadijah, what has happened to me?" He (peace and blessings be upon him) told her the whole incident and said: "I have come to fear for my life." She said: "Never! Rejoice, for by Allah, Allah will never disgrace you. You maintain the ties of kinship, you speak the truth, you bear the burden of the helpless, you honour the guest, and you help people in the calamities that befall them in the way of truth." Then Sayyidah Khadijah took him (peace and blessings be upon him) to Waraqah bin Nawfal bin Asad bin Abd al-Uzza bin Qusayy, who was the paternal uncle of Sayyidah Khadijah, her father's brother. He had become a Christian in the period of Jahiliyyah. He knew how to write Arabic, and he used to write in Arabic as much of the Injil as Allah willed. He was very old and had become blind. Sayyidah Khadijah said: "O my uncle, listen to your nephew." Waraqah bin Nawfal asked: "O my nephew, what do you see?" So the Messenger of Allah (peace and blessings be upon him) told him what he had seen. Waraqah said: "This is the same Namus, that is, the angel, who was sent down to Musa (peace be upon him)."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it with this wording.

It is narrated from Imam al-Zuhri that the first person among women to believe in the Messenger of Allah (peace and blessings be upon him) was Sayyidah Khadijah (may Allah be pleased with her).