Bukhari

Hadith Commentary:
It is understood from this that it is prohibited to hang a bell or any other musical instrument around the neck of an animal merely for adornment or pride.

Ibn al-Jawzi said: Regarding the meaning of "strings" (awtar), there are three opinions. The first is that they used to hang bowstrings around the necks of camels, believing that this would protect them from the evil eye. They were commanded to cut them off to make it clear that strings cannot avert anything decreed by Allah.

That is, the first opinion is that the ignorant among the Arabs would hang some kind of string as an amulet (ta‘widh) around the necks of camels so that they would not be afflicted by the evil eye. Therefore, they were commanded to cut and throw them away, so that they would know that these cannot repel Allah’s decree.

The second opinion is that hanging such strings, etc., around the necks of animals was prohibited out of fear that they might become tight around their necks and strangle them, or get entangled in a tree and cause harm, thereby causing suffering to the animals.

The third opinion is that they would hang bells, whereas in places where ringing bells are present, the angels of mercy do not enter.

Imam al-Bukhari rahimahullah has alluded to the narration reported by al-Daraqutni rahimahullah regarding this hadith, in which it is stated explicitly:

«لاَ يَبْقَيَنَّ فِي رَقَبَةِ بَعِيرٍ قِلاَدَةٌ مِنْ وَتَرٍ، أَوْ قِلاَدَةٌ إِلَّا قُطِعَتْ» "except for cutting"—that is, whether it is a string or a bell around the neck of any animal, they should not be left (but rather cut off).

(Fath al-Bari)

Hadith Commentary:
The scholars of hadith have mentioned several opinions regarding this prohibitive command.

➊ People used to tie a string or the like around the necks of camels so that they would not be afflicted by the evil eye. The Messenger of Allah (sallallahu alayhi wa sallam) commanded that these be cut off, because such actions corrupt one’s creed, and doing so cannot repel Allah’s decree.

➋ People would tie a bell to this string, as is mentioned in a hadith:
“The angels do not accompany a caravan in which there are bells ringing.”
(Sunan Abi Dawud, Kitab al-Jihad, Hadith: 2554)
Moreover, bells and the like are contrary to the tactics of war.

➌ There is a risk that such a string could cut the animal’s neck; sometimes it causes difficulty in breathing and becomes a hindrance when grazing. It is also possible that the string could get caught on a tree, causing the animal’s neck to be cut. For these reasons, the Messenger of Allah (sallallahu alayhi wa sallam) issued the aforementioned prohibitive command.

From the heading of Imam al-Bukhari (rahimahullah), it appears that he preferred the second explanation.
(Fath al-Bari: 6/172)
And Allah knows best.

Hadith Commentary:

Benefits and Issues:
➊ In the era of ignorance (Jahiliyyah), people used to put a string necklace around the necks of animals, especially camels, and believed that it had the property of protecting from the evil eye. Therefore, the Prophet (sallallahu alayhi wa sallam) ordered it to be cut off, as it has no role in protecting from the evil eye.
➋ Some scholars are of the opinion that it causes discomfort to the animal, making grazing and breathing difficult, and there is also the risk of the animal getting strangled if it gets caught in a tree.
➌ Some believe that a bell was tied to it. If the necklace is loose, poses no risk of any kind, and is merely for adornment, then according to Imam Nawawi, it is permissible. According to Allamah Ayni, if it is for the purpose of an amulet (ta‘widh) and contains a verse of the Qur’an, or the name of Allah, with the intention of seeking blessing or seeking refuge in Allah’s names and His remembrance, then it is not prohibited. Similarly, if it is for adornment while avoiding excess and extravagance, then it is also not prohibited.
(‘Umdat al-Qari, vol. 7, p. 43)
However, it is evident that since the Messenger of Allah (sallallahu alayhi wa sallam) did not place Qur’anic verses or the names of Allah around the necks of animals, we too should avoid this practice.

Benefits and Issues:
Allamah Khattabi writes that Imam Malik gives the reason that people used to put it (the string) as an amulet for protection against the evil eye.
And they considered it to be effective in itself.
Some scholars are of the opinion that people used to tie these around their necks to hang bells on them.
Some have said that it was to prevent the animal from being strangled while running.
In any case, whatever the reason may be, tying a string has been prohibited.
And similarly, it is not permissible to use other ignorant amulets and charms either.

Hadith Authentication: [وأخرجه البخاري 3005، ومسلم 2115، من حديث مالك به]
Jurisprudential Explanation:
➊ Hanging threads, beads, etc., with the belief that illness will not afflict or that it will provide protection from the evil eye, is not permissible. However, regarding hanging written Qur’anic verses or non-shirk (non-polytheistic) phrases, there is a difference of opinion among the pious predecessors (salaf saliheen). Sayyiduna Sa’id ibn al-Musayyib rahimahullah considered it permissible. See [السنن الكبريٰ للبيهقي 351/9 وسنده صحيح]. But it is better to abstain from even these.
◄ Ibrahim al-Nakha’i rahimahullah considered amulets (ta’widh) for children to be disliked (makruh) because they might enter the toilet with them. [مصنف ابن ابي شيبه 376/7 ح 23466 وسنده صحيح، دوسرا نسخه 16/8 ح 23823،]
➋ Ishaq ibn Mansur al-Kawsaj rahimahullah asked Imam Ahmad ibn Hanbal rahimahullah about hanging the Qur’an (as an amulet), and he replied: Hanging anything (written for treatment) is disliked (makruh). [ديكهئے مسائل اسحاق و احمد ج 1 ص 193 فقره 382، التمهيد 164/17]. The preferred opinion is that Qur’anic and non-shirk amulets are not shirk or innovation (bid’ah), but as a preventive measure (sadd al-dhara’i’), even these amulets should not be worn.
➌ It is necessary to avoid doubtful and suspicious things.
➍ The effect of the evil eye is real. See [صحيح بخاري 5740 و صحيح مسلم 2187]. But its treatment is not amulets and charms, rather it is the prescribed (masnun) supplications. For example, the supplication mentioned in «أَعُوذُ بِكَلِمَاتِ اللَّهِ التَّامَّةِ مِنْ كُلِّ شَيْطَانٍ وَهَامَّةٍ وَمِنْ كُلِّ عَيْنٍ لَامَّةٍ». See [صحيح بخاري 3371].