Revelation came to the Prophet (peace and blessings be upon him) when he was forty-three

It is narrated from Qubath bin Ashyam al-Kinani al-Laythi (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) was made a Prophet exactly forty years after the Year of the Elephant.
This hadith has a sahih (authentic) chain, but the two Shaykhs (Bukhari and Muslim) did not record it. Imam Bukhari recorded the hadith of Ikrimah from Ibn Abbas (may Allah be pleased with them both) that he (peace and blessings be upon him) was sent at the age of forty. And the proof of the soundness of the hadith of Qubath bin Ashyam is that the leader of the Tabi'in adopted this very view.

It is narrated from Sayyiduna Sa'id bin al-Musayyib (may Allah be pleased with him); he said: (Revelation) came down upon the Prophet (peace and blessings be upon him) when his blessed age was forty-three years.

It is narrated from Sayyidah Khadijah (may Allah be pleased with her); she said: When the coming of revelation to the Messenger of Allah (peace and blessings be upon him) was delayed, he (peace and blessings be upon him) was deeply grieved by it. Seeing his grief, I said: "Your Lord has left you alone (as a trial) because of the distress He sees in you." Then Allah Almighty revealed this verse: "(O Prophet,) your Lord has neither forsaken you nor is He displeased with you." [Al-Duha: 3]
This hadith has a sahih (authentic) chain, but the two Shaykhs (Bukhari and Muslim) did not record it because of irsal (a missing Companion link) in it.

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said to Jibril (peace be upon him): "Why do you not visit us more often than you visit us now?" Then Allah, the Mighty and Majestic, revealed the verses from "And we do not descend except by the command of your Lord" up to "and your Lord is never forgetful" [Maryam: 64]: "And (Jibril said:) We do not descend except by the command of your Lord, and your Lord is never forgetful."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim).

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both); he said: The Noble Qur'an was separated from the Dhikr (the Preserved Tablet) and placed in the House of Honour (Bayt al-Izzah) in the lowest heaven. Then Jibril (peace be upon him) began to send it down upon the Prophet (peace and blessings be upon him) little by little, in measured portions. Sufyan says: Five verses or about that would come down (at a time).
This hadith has a sahih (authentic) chain, but the two Shaykhs (Bukhari and Muslim) did not record it.