Does the time for Maghrib prayer begin when the sun sets behind the mountain?

The questioner : This, by Allah, is a matter that bothers me regarding the adhan. For example, if one is in a flat area and the sun sets behind a mountain, does that mean I am done when it sets behind the mountain?

The Shaykh : And you are saying this?

The questioner : No, I mean logically, no.

The Shaykh : You are a teacher.

The questioner : I am telling you, logically, no.

The Shaykh : So the Shari'ah is in accordance with logic.
The student : Does the Shari'ah prescribe it? Tell him, does the Shari'ah prescribe it?

The questioner : Or does the Shari'ah prescribe it?

The Shaykh : If it is prescribed for him to remember Allah, it is prescribed for him to remember the Messenger of Allah (peace and blessings be upon him). And if it is not prescribed for him to do the first, it is not prescribed for him to do the second. And the answer is:
The student : Is that clear?

The Shaykh : I do not think it is clear.
The student : Now it will become clear.

The Shaykh : Now it will become clear. It is not permissible.
The student : It is not permissible.