Is it narrated from the Messenger of Allah, peace and blessings be upon him, "Do not trust them, even if you have lived with them for forty years," and if it is authentic, what were the circumstances in which it was said?

The phrase in question has not been found in the books of Hadith, and it is not likely to be authentically attributed to the Prophet, peace be upon him. Rather, it contradicts what has been authentically reported from him: that the best of women is she who pleases him when he looks at her, obeys him when he commands, and does not contradict him regarding herself or his wealth in a way he dislikes. This is also supported by other narrations stating that the best benefit God has bestowed upon a Muslim is a righteous wife who pleases him when he looks at her, protects herself in his absence, and obeys him when he commands her.