Chapter: Safeguarding Secrets
Allah the Exalted says, “And fulfil the covenant, for the covenant will be asked about.” (Qur’an 17:34)

685 – Abu Saʿid al-Khudri (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) said, “Among the worst of people in Allah’s sight on the Day of Resurrection is a man who has relations with his wife and she with him, and then he spreads her secret.” Reported by Muslim.

Explanation
Imam al-Nawawi (may Allah have mercy on him) writes under the heading “Safeguarding Secrets”: A secret is whatever takes place privately between you and your companion. It is unlawful for you to divulge or disclose that secret to anyone—whether he plainly says, “Do not tell anyone; this is a trust,” or you understand from a practical sign that he does not wish anyone to know, or you gather from the surrounding circumstances that he does not want it revealed.

First example, an explicit statement: someone relates something to you and then says, “Do not inform anyone; it is a trust with you.”
Second example, a practical indication: while telling you, he keeps glancing about