Is there an agreed-upon Hadith from the Prophet (peace be upon him) that mentions his marriage to Aisha when she was nine years old, and are the sayings of Lady Aisha considered part of the Sunnah?

There is no prophetic text regarding the marriage of Aisha, may Allah be pleased with her, but rather she herself narrated it, and its chain of narration is authentic. The books of Hadith and Sira (Prophetic biography) have successively reported that she married at the age of six and the Messenger of Allah, peace and blessings be upon him, consummated the marriage with her when she was nine years old. This is what Al-Bukhari and Muslim, the two Sheikhs, agreed upon, and the Ummah accepted it. Furthermore, Aisha is the source of the story, which necessitates believing her statement.