Short answer
The first sin against Allah was Iblis's arrogance: he refused to bow to Adam when Allah commanded, saying "I am better than him". The first human slip was Adam and Hawwa eating from the forbidden tree, which they repented of and were forgiven. The first murder on earth was one of Adam's sons killing his brother.
Sunni scholars note that Iblis's sin was pride joined with rejecting Allah's command, which made it disbelief, while Adam's was a slip followed at once by repentance. The difference shows that persisting in a sin out of arrogance is far worse than falling into it and turning back.
The four schools agree on this.
Explanation
When Allah commanded the angels to bow to Adam, "they prostrated, except for Iblees. He refused and was arrogant and became of the disbelievers" (2:34). Asked why, he said: "I am better than him. You created me from fire and created him from clay" (7:12). He compared, judged himself superior, and rejected the command. Scholars call this the root of all sin: pride, and setting your own opinion against Allah's command.
Then Adam and his wife ate from the tree, after Shaytan swore he was advising them. Their response was the opposite of Iblis's: "Our Lord, we have wronged ourselves, and if You do not forgive us and have mercy upon us, we will surely be among the losers" (7:23). Allah forgave them. One sin led to a curse because its owner insisted on it; the other led to forgiveness because its owners repented.
The first crime between people was the story of Adam's two sons (5:27-31). Both offered a sacrifice; Allah accepted it from the righteous one, and the other, out of envy, killed his brother. The commentators name them Qabil and Habil (Cain and Abel), though the Quran does not. The Prophet ﷺ said that no soul is killed unjustly except that the first son of Adam bears a share of its blood, because he was the first to set the practice of killing.
The lessons: beware of pride and envy, and turn back quickly when you slip. See is pride a sin?, did Adam and Eve sin? and is Shaytan an angel?
The evidence
Quran, Surah Al-Baqara 2:34
وَإِذْ قُلْنَا لِلْمَلَٰٓئِكَةِ ٱسْجُدُوا۟ لِءَادَمَ فَسَجَدُوٓا۟ إِلَّآ إِبْلِيسَ أَبَىٰ وَٱسْتَكْبَرَ وَكَانَ مِنَ ٱلْكَٰفِرِينَ ﴿٣٤﴾
And [mention] when We said to the angels, "Prostrate before Adam"; so they prostrated, except for Iblees. He refused and was arrogant and became of the disbelievers.
Records Iblis's refusal, the first sin.
Quran, Surah Al-A'raaf 7:12
قَالَ مَا مَنَعَكَ أَلَّا تَسْجُدَ إِذْ أَمَرْتُكَ ۖ قَالَ أَنَا۠ خَيْرٌۭ مِّنْهُ خَلَقْتَنِى مِن نَّارٍۢ وَخَلَقْتَهُۥ مِن طِينٍۢ ﴿١٢﴾
[Allah] said, "What prevented you from prostrating when I commanded you?" [Satan] said, "I am better than him. You created me from fire and created him from clay."
Shows the pride behind Iblis's refusal.
Quran, Surah Al-Maaida 5:30
فَطَوَّعَتْ لَهُۥ نَفْسُهُۥ قَتْلَ أَخِيهِ فَقَتَلَهُۥ فَأَصْبَحَ مِنَ ٱلْخَٰسِرِينَ ﴿٣٠﴾
And his soul permitted to him the murder of his brother, so he killed him and became among the losers.
Records the first murder.
Sahih al-Bukhari 3335
Sahih... لاَ تُقْتَلُ نَفْسٌ ظُلْمًا إِلاَّ كَانَ عَلَى ابْنِ آدَمَ الأَوَّلِ كِفْلٌ مِنْ دَمِهَا ...
"No soul is killed unjustly except that the first son of Adam bears a share of its blood."
Shows the first murderer shares the burden of every murder after him.
Where the ruling is recorded
- Tafsir Ibn Kathir, Commentary on Surah al-Baqarah, verse 34, and Surah al-Ma'idah, verses 27 to 31. Explains Iblis's refusal and the story of Adam's two sons.
- al-Fiqh al-Akbar (Imam Abu Hanifa), Section on the prophets. States that slips can occur from the prophets, unlike deliberate sins.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.