The crescent is not a symbol of Islam because it is not authentic from the Prophet (ﷺ) that he and his Companions used it to represent Islam. There is a weak narration that suggest that the flag of the Prophet (ﷺ) had a white crescent.
Hafidh Ibn Hajr rahimahullah wrote:
قال بن يونس وفد على النبي صلى الله عليه و سلم وعقد له راية على قومه سوداء فيها هلال أبيض وشهد فتح مصر وله بها عقب
“Ibn Yunus said: [Abu Kunud] came to the Prophet (ﷺ) as part of delegation. So he gave them black flag with a white crescent and he witnessed the conquest of Egypt and he had with it a following”.
[Isaabah fi Tamyeez as-Sahabah no. 3194 (3/72)]
This narration is weak because Ibn Yunus is a student of the Tabi’een and he did not meet the Prophet (ﷺ). Hafidh Ibn Hajr also says it was reported by Sa’eed ibn Ufair from Amr ibn Zuhair ibn Asmar ibn Abu al-Kunud. Sa’eed Ibn Ufair is not a Tab’ieen nor is ‘Amr ibn Zuhair. So this narration is disconnected.
It is also reported in Tareekh ad-Damashq from Wahshi ibn Harb (may Allah be pleased with him) that :
أَنَّهُ وَفَدَ عَلَى رَسُولِ اللَّهِ ـ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ـ فِي اثْنَيْنِ وَسَبْعِينَ رَجُلا مِنَ الْحَبَشَةِ ، وَأَنَّ النَّبِيَّ ـ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ـ قَوَّدَنِي عَلَيْهِمْ وَعَقَدَ لِي رَايَةً صَفْرَاءَ ، ذِرَاعَيْنِ فِي ذِرَاعَيْنِ ، وَفِيهَا هِلالٌ أَبْيَضُ ، وَعَذَبَتَانِ سَوْدَاوَانِ
“He came to the Prophet in a delegation of 72 men from Abyssinia and the Prophet (ﷺ) appointed me the leader of them and granted me a yellow flag, one arm-span by one arm-span in length, and on it was a white crescent and two black tassels…
[Tareekh ad-Damashq (62?414)]
However the narration is also weak due to the presence of unknown narrators like Ishaq ibn Harb ibn Wahshi.
The Prophet’s (ﷺ) flag (رَايَةُ) used to be black and his standard (لواء ) used to be white:
عَنِ ابْنِ عَبَّاسٍ، قَالَ كَانَتْ رَايَةُ رَسُولِ اللَّهِ صلى الله عليه وسلم سَوْدَاءَ وَلِوَاؤُهُ أَبْيَضَ
Narrated Ibn ‘Abbas (may Allah be pleased with him)
“The flag of the Messenger of Allah (ﷺ) was black, and his standard was white.”
[Saheeh Tirmidhi no. 1681]
Ibn al-‘Arabi rahimahullah said:
“The standard (لواء ) is other than flag (رَايَةُ). The standard (لواء ) is tied to the end of a spear and is turned [left and right]. The (رَايَةُ) is what is tied to the end of a spear and left [in the ground] until it moves with the winds”
So the flag of the Prophet (ﷺ) did not use a crescent nor is it known that this symbol was used by him or his Companions. In fact, it was a symbol used by the Ottoman Empire.
Kamil ibn Hussain (1351H ) wrote:
“The Seljuks and the Uthmaniyyah are ascribed to one father and he is Ughir Khan ibn Qarrah Khan. He was the first to issue laws for the Turks and take concern for their city. In general, from his effects/remnants was the crescent which was the symbol for the Uthmaani state…”
[Nahr adh-Dhahab fi Tareekh al-Halab (3/109)]
It was also said that the crescent was a counterpart to the Red Cross of the Christians.
So like that Shaikh Ibn ul-‘Uthaymeen rahihamullah said:
“So like this, it is not necessary to place crescents at the tops of minarets due to this doubt and due to what it involves of wasting money and time” [Majmoo’ al-Fatawa wa-Rasaail (16/178)]
In sum, it is not a symbol of Islam
And Allah Knows Best
Faisal Ibn Abdul Qaadir Ibn Hassan