Tafsir of Al-Fajr 89:9

"And Thamud" is a conjunctive [‘atf] referring back to ‘Ad. They are a well-known tribe named after their forefather, Thamud, the brother of Jadis—both being descendants of ‘Abir ibn Iram ibn Sam ibn Nuh, peace be upon him. They were among the ‘Arab al-‘Ariba (the pure Arabs) who inhabited al-Hijr, located between the Hijaz and Tabuk, and they worshipped idols.

The prohibition of sarf (diptote declension) for the word "Thamud" is due to it being a proper noun (al-‘alamiyyah) and feminine gender (al-ta’nith). Ibn Wathab recited it with tanwin (diptote to triptote), treating it as referring to the "tribe" (al-hayy). Some have said this, and it is apparent that the name is Arabic; it has been explicitly stated that it is a fa‘ul form derived from al-thamd, which refers to little water that has no source or replenishment. From this comes the expression "such-and-such is mathmud," meaning women have depleted his seed due to his frequent intercourse with them, or one who is mathmud if he is asked so frequently that his wealth is depleted. Al-Raghib narrated that it is non-Arabic (‘ajami), and thus the prohibition of sarf is due to being a proper noun and foreign origin.

"Who carved the rocks"—that is, they cut through the rocks of the mountains and made houses within them, hewing them out of the stone, as in the Almighty’s saying: "And you carve houses from the mountains." It is said that the first to carve stone, rock, and marble were the Thamud, and they built one thousand seven hundred cities, all made of stone, though I doubt the authenticity of this.

"In the valley" refers to Wadi al-Qura. It was also read with a ya at the end of the letters (i.e., bil-wadi). The ba denotes container-ship (zarfiyyah), and the prepositional phrase relates to "carved," or to an implied state (hal) of the subject or object. It has been said that the ba indicates an instrument or causality, relating to "carved"—meaning they carved the rock by means of their valley, or because of it; meaning they cut the rock and split it, making it a valley and a reservoir for their water, a deed of those possessing might and ambition. This, however, is contrary to the apparent meaning. Regardless, al-jawb means cutting. The apparent meaning is that it is literal; you say "I traversed (jubtu) the lands and crossed them (ajubuha)" if you cut across them. The poet said: "I have not seen a she-camel before this one, bearing sixty loads, that has traversed (jabat) a land." It is also called jawab (an answer) because it cuts off the question. Al-Raghib said: Al-jawb is the cutting of a jawbah, which is a depression in the earth, then it is used for cutting across any land. The jawab (answer) to speech is that which cuts through the void, reaching the listener's ear from the speaker's mouth, but it is specifically applied to the returning speech, not the initial address. Choose for yourself what pleases you.