What biblical passage(s) reveal(s) what God says was the fallen angels' sin? From this question: Does the Bible provide any passages that might help in understanding how Satan became sinful?, answers have attempted to find from a biblical-basis, how Satan became sinful.

This question is asking what God himself says was the sin of the fallen angels. The earlier question deals with how, this one: According to God, what was the sin of the fallen angels?
God's message to King Saul through prophet Samuel, after Saul disobeys God, is:


  1 Samuel 15:22-23
  (RSVCE)
  22 And Samuel said,   “Has the Lord as great delight in burnt
  offerings and sacrifices,
      as in obeying the voice of the Lord? Behold, to obey is better than sacrifice,
      and to hearken than the fat of rams. 23 For rebellion is as the sin of divination,
      and stubbornness is as iniquity and idolatry. Because you have rejected the word of the Lord,
      he has also rejected you from being king.”[a]


Footnotes:
a. 15.22-23 Samuel does not condemn sacrifices but says obedience is better.


  The Jerusalem Bible | Popular Edition has for v. 23: "Rebellion is a sin of sorcery,  presumption a crime of the teraphim."[b]


Footnotes:
b. 15.23 'False gods': teraphim - people kept these things in their houses?! Scarey! - were household idols.



It is assumed that the readers of this answer know the connection between divination, idolatry, sorcery, idols, and demons/fallen angels.



Another revealing is the passage is John 8:39-47 (RSVCE): Jesus and Abraham in which Jesus contrasts children of the devil with children of Abraham. Thus Abraham did the opposite of what the devil did. Abraham listened to God, obeyed, trusted, and had faith in God.



Recalling Jesus saying, “If you love me, you will keep my commandments." [John 14:15 (RSVCE)] and our first parents fell by failing to keep the one commandment God enjoined on them, and Abraham passing the obedience test, and the passage quoted in 1 Samuel above, loving God means keeping his commandments and obeying him especially when he directly commands.

Combining this with the passages from 1 Samuel 15 and John 8 above, the sin of the fallen angels was that of disobedience.