与子成说只和一人至死不渝只信一人
[yŭ zi chéng shuō zhĭ hé yī rén zhì sĭ bù yú zhĭ xìn yī rén]
This phrase translates to pledging loyalty to only one person until death. It indicates unwavering trust and fidelity in a personal or romantic relationship with one special person.