Is it valid to combine the two prayers because of dust, and when is combining prescribed?

Question:
He says: Honorable Shaykh, may Allah grant you success. The imam of a mosque combined the midday and afternoon prayers for us last Tuesday because of the dust, and we prayed with him. He cited to us the hadith of Ibn 'Abbas that the Prophet (peace and blessings be upon him) combined the two prayers, in which Ibn 'Abbas said he did so so as not to burden his Ummah. The question is: Was the imam's action valid? What is the ruling on our prayer? And do we have to repeat it?

Answer:
The imam's action is not valid, and there is no blame in this. Dust is a common occurrence, and it used to happen among the Muslims; it is nothing new. It happens among the Muslims, and there is no evidence that they combined prayers because of dust. Combining is only for one of three things: either travel that permits shortening the prayer, or illness, or rain that wets the clothes and results in mud on the road between them and the mosque, and this is specifically between the sunset and night prayers. These are the conditions for combining.
As for dust, this imam has to bring an explicit text that the Muslims combined prayers, or evidence that dust is an excuse for combining. And he has to repeat the prayer; because the default is that every prayer is performed in its own time, and it is not permissible to combine without an excuse. So they have to repeat it. Yes.