Short answer
Scholars differ. A sahih hadith says the first thing Allah created was the Pen, which He commanded to write everything until the Hour. Other sahih hadith say His Throne was on the water when He wrote the decrees, so many scholars hold that the Throne and water came before the Pen. Before anything, only Allah existed.
Sunni creed holds that Allah alone is eternal and everything else was created after not existing. Which created thing came first is a question of scholarship, not a point of iman. The popular report that the light of the Prophet ﷺ was the first creation is not found with a reliable chain in the hadith collections.
The four schools agree on this.
Explanation
The Prophet ﷺ said: "Allah existed and there was nothing other than Him, and His Throne was on the water, and He wrote everything in the Reminder, and He created the heavens and the earth" (Sahih al-Bukhari). The Quran agrees that "His Throne had been upon water" before the heavens and earth were created (11:7).
He also said: "The first thing Allah created was the Pen. He said to it: Write. It said: My Lord, what shall I write? He said: Write the decrees of all things until the Hour" (Abu Dawud, graded sahih). And in Sahih Muslim, Allah wrote the decrees fifty thousand years before creating the heavens and the earth, "and His Throne was on the water".
How do these fit? Many scholars said the Throne and water existed before the Pen, and the Pen was the first thing created for this world's decree. Others read the Pen hadith literally and said it was the first of all. The question has no practical effect on belief, so you can hold either view.
Many people have heard that the first thing Allah created was the light (nur) of Muhammad ﷺ, quoted as a long report from Jabir and often attributed to the Musannaf of Abd al-Razzaq. It is not found with a chain in the known hadith collections, and hadith scholars such as al-Suyuti said it has no chain that can be relied on. Some respected scholars, including South Asian elders, quoted it in books on his virtues, and all Sunnis hold that his rank is the highest in creation. That rank is proven by sound reports: asked when prophethood was made his, he said, "While Adam was between the spirit and the body" (Tirmidhi, graded sahih). The calm position is to affirm what the sound reports say and not to make the Jabir report a point of belief. See How was the world created in Islam?
The evidence
Quran, Surah Hud 11:7
وَهُوَ ٱلَّذِى خَلَقَ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضَ فِى سِتَّةِ أَيَّامٍۢ وَكَانَ عَرْشُهُۥ عَلَى ٱلْمَآءِ لِيَبْلُوَكُمْ أَيُّكُمْ أَحْسَنُ عَمَلًۭا ۗ وَلَئِن قُلْتَ إِنَّكُم مَّبْعُوثُونَ مِنۢ بَعْدِ ٱلْمَوْتِ لَيَقُولَنَّ ٱلَّذِينَ كَفَرُوٓا۟ إِنْ هَٰذَآ إِلَّا سِحْرٌۭ مُّبِينٌۭ ﴿٧﴾
And it is He who created the heavens and the earth in six days - and His Throne had been upon water - that He might test you as to which of you is best in deed. But if you say, "Indeed, you are resurrected after death," those who disbelieve will surely say, "This is not but obvious magic."
The Throne was on the water before the heavens and earth.
Sahih al-Bukhari 3191
Sahih... كان الله ولم يكن شيء غيره وكان عرشه على الماء ...
"Allah existed and there was nothing other than Him, and His Throne was on the water."
Only Allah is eternal; the Throne and water came early in creation.
Sunan Abi Dawud 4700
Grading to be checked... إن أول ما خلق الله القلم فقال له اكتب ...
"The first thing Allah created was the Pen, and He said to it: Write."
The hadith on the Pen as first created.
Sahih Muslim 2653
Sahih... كتب الله مقادير الخلائق قبل أن يخلق السموات والأرض بخمسين ألف سنة ...
"Allah wrote the measures of creation fifty thousand years before creating the heavens and the earth."
The decree was written while the Throne was on the water.
Jami' at-Tirmidhi 3609
Grading to be checked... متى وجبت لك النبوة قال وآدم بين الروح والجسد ...
"They asked: Messenger of Allah, when was prophethood made yours? He said: While Adam was between the spirit and the body."
Shows the Prophet's ﷺ high rank before creation from a sound report, without relying on the Jabir wording.
Where the ruling is recorded
- al-Fiqh al-Akbar (Imam Abu Hanifa), Section on Allah's eternal attributes. Affirms that Allah is eternal and everything else is created.
- Sharh al-Aqa'id al-Nasafiyya (al-Taftazani), Section on the world being originated. Proves that everything other than Allah came into being after not existing.
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.