Does what happened with the wife constitute marital infidelity, and is the husband entitled to make her swear by Allah on the Quran, or by way of Li'an (mutual imprecation), to find out if she is in contact with another man while under his marital authority?

The fundamental principle concerning a Muslim is their integrity, and it is not permissible to assume otherwise without clear evidence. This is because Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." You have suspected your wife unjustly and investigated, which led to espionage and deception.

It has been narrated from the Prophet, peace and blessings be upon him: "If you suspect, do not investigate," and this Hadith has supporting evidence.

There is nothing to prove your wife's betrayal. Among her rights upon you is to protect her and safeguard her honor from anything that might tarnish it, but without excessive monitoring of her movements or negative assumptions that lead to dire consequences.

It is better to raise her on faith, protect her, keep her away from temptations, and maintain her residence with you. It is not permissible to question her about suspicious relationships or demand an oath with her hand on the Qur'an. We advise you to hold onto your wife and treat her kindly.