Tafsir of Al-Jathiyah 45:18

"Then We placed you upon a Shari'ah": Meaning a path and a methodology; it is derived from 'shar'ahu' when one initiates or establishes it for others to follow. In Al-Bahr, it is noted that 'Shari'ah' in the speech of the Arabs refers to the place from which people draw water from rivers and the like. Therefore, the Shari'ah of religion is derived from this, in that people draw from it the command of Allah, His mercy, and proximity to Him, the Almighty and Majestic.

Al-Raghib said: Shar' is a verbal noun, then it was made into a name for an open, manifest path, and it is called shar', shir'ah, and shari'ah. This was then metaphorically applied to the divine methodology of religion. He further stated: Some have said it was named Shari'ah in analogy to the Shari'ah (watering place) of water, in the sense that whoever enters into it with truth and sincerity is quenched and purified. By "quenched," I refer to what some sages said: "I used to drink and was not quenched; but when I knew Allah, the Almighty and Majestic, I was quenched without drinking." By "purified," I refer to what the Almighty and Majestic said: "Allah only intends to remove from you the impurity [of sin], O people of the [Prophet's] household, and to purify you with [extensive] purification."

The apparent meaning here is the linguistic one, and the tanwin (indefinite marking) denotes magnification; meaning, a Shari'ah of great status.

"From the matter": Meaning the matter of religion. Abu Hayyan permitted it to be the verbal noun of 'commanding,' with the intended meaning being [that of] command and prohibition, but this is as you see it [i.e., questionable].

"So follow it, and do not follow the inclinations of those who do not know."

Meaning: The opinions of the ignorant that follow their desires. The intended meaning is inclusive of every misguided person. It has been said: They are the Jahalah (ignorant ones) of [the tribes of] Qurayzah and Nadir. It has also been said: They are the leaders of the Quraysh, who used to say to him, may Allah bless him and grant him peace: "Return to the religion of your forefathers."