I had sexual intercourse with my wife while believing that her menses stopped but after intercourse I discovered that she is not yet pure, do I have to expiate for this?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If you had intercourse with your wife while believing that she became pure from menses and after she performed Ghusl (ritual bath), but after intercourse she had some blood discharges, then there is no harm on you. However, if you had intercourse with her before she had Ghusl, then you have to seek forgiveness from Allaah as it was not permissible for you to have intercourse with her until she had Ghusl after her menses stopped and became pure.

Allaah Knows best.