In regards to not cutting one’s hair and pairing one’s nails, Rasūlullah صلى الله عليه و سلم has mentioned, “When the ten days (of Dhul Hijjah) commence and a person has a sacrificial animal, which he intends to slaughter, then let him not cut his hair or pare his nails.” (Muslim)
قال إذا دخل العشر وعنده أضحية يريد أن يضحي فلا يأخذن شعرا ولا يقلمن ظفرا
(مسلم رقم الحديث 1977, دار إحياء التراث العربي)
Rasūlullah صلى الله عليه و سلم did not mention the exact reason why a person should not cut his hair or nails; therefore, any explanation for its underlying reason is mere conjecture. Nevertheless, some ‘Ulamā and commentators of Hadīth state various possible reasons for this instruction. Imām Nawawī رحمه الله states that the wisdom behind not cutting the hair and trimming the nails is to include these body parts in the emancipation from the fire after the sacrifice of the animal along with the remainder of the body. This understanding is possibly based on the following narration quoted in al-Targhīb wa al-Tarhīb, “Whoever offers a sacrifice with a contented heart anticipating reward for his sacrifice, it will serve as a veil for him from the fire.”
وروي عن الحسين بن علي رضي الله عنهما قال قال رسول الله صلى الله عليه و سلم من ضحى طيبة نفسه محتسبا لاضحيته كانت له حجابا من النار (الترغيب والترهيب رقم الحديث 1664, دار الكتب العلمية)
Other commentators of Hadīth such as Ibn Qayyim رحمه الله state that it constitutes part of the completion of the sacrificial rite of the Qurbānī. He presents the following narration of Rasūlullah صلى الله عليه و سلم as evidence for this claim, “Cut your hair and nails, trim your mustache and shave your pubic hairs for verily that constitutes the completion of your sacrifice by Allah, the Most-Honored and Sublime”
تأخذ من شعرك وأظفارك وتقص شاربك وتحلق عانتك فتلك تمام أضحيتك عند الله عزوجل
(ابو داود رقم الحديث 2789 , دار الفكر)
Imām Suyūtī رحمه الله mentions that some ‘Ulamā are of the opinion that the wisdom behind not cutting one’s hair and nails is to resemble, in some way or the other, the pilgrims of Hajj.
Despite the various possible wisdoms, the main underlying reason for a Muslim should be to carry out the instructions and desires of our beloved Nabī صلى الله عليه و سلم . In the realm of love, worship and obedience, one’s logic has no stake. Allah and His Nabī صلى الله عليه و سلم know best why we were instructed to refrain from cutting our hair and nails. Therefore, as dutiful slaves we should carry out the instruction in a spirit of undying devotion.