Wetness and stains on sofa went away without washing

Asalamalikum,Hadhrat Mufti Sahab, about a year ago I had wet dream in morning while lying on a sofa. Waking up I saw the stains but didn’t know the concept of najasat that it transfer too – I didn’t wash it and after some time it dried and then after some time stains were also gone. So my question is:
Can we lie on that sofa?