Before electrons, neutrons and protons were discovered the Atom was believed to be the smalest particle on earth. However, later researach tells us that Atmos contains of electrons, neutrons and protons i.e. their weights and lately it turned out that those neutrons and protons are made of quarks.

Quran mentions it about 1400 years ago.

[Quran 10:61]
“Whatever you may be doing, and whatever portion you may be reciting from the Quran, and whatever deed you may be doing, We [Allah] do witness when you are doing it. And nothing is hidden from your Lord (so much as) the weight of an atom on Earth and in heaven, not less than that nor more but is (written) in a clear record (the Preserved Tablet).”

And in [Quran 34:3]

“Those who disbelieve say: The Hour will never come unto us. Say: Nay, by my Lord, but it is coming unto you surely. (He is) the Knower of the Unseen. Not an atom’s weight, or less than that or greater, escapeth Him in the heavens or in the earth, but it is in a clear Record,”

From above we see actually Atom itself is made up electrons, neutrons and protons and thus Atom is not the smallest particle but contains weight.
The electrons, neutrons and protons were discovered by

The Nucleus of Atom was discovered in 1911 by Ernest Rutherford. where as:

Neutron: James chadwick (1932)
Proton: Ernest Rutherford (1918)
Electron: John Joseph Thomson (1897)
Quraks: Murray Gell-Mann (1969)

How could an illiterate man who lived 1400 years ago have known that the atom is not the smallest component of matter?

“..  But only men of understanding will pay heed.” [Quran 39:9]