How do we respond to those who say that we pray Maghrib while the sun has not yet set?

The beginning of the Maghrib prayer time is at sunset. The proof for this is that the Prophet, peace and blessings be upon him, used to pray Maghrib when the sun set and disappeared behind the veil. When it "wajib" — meaning the sun had set and disappeared from sight. The conclusion is that it is not permissible to pray Maghrib before sunset, and whoever prays it before then must repeat it.