Musnad Ahmad 31:664

Isma'il ibn Muhammad—meaning Abu Ibrahim al-Mu'aqqib—narrated to us, Marwan—meaning ibn Mu'awiyah al-Fazari—narrated to us, 'Umar ibn Hamzah al-'Umari narrated to us, 'Abd al-Rahman ibn Sa'd, the freed slave of the family of Abu Sufyan, narrated to us: I heard Abu Sa'id al-Khudri say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, among the greatest of trusts in the sight of Allah on the Day of Resurrection is a man who has intimate relations with his wife, and she has intimate relations with him, then he spreads her secret."