Refrain from cutting your hair or trimming your nails until you have sacrificed an animal during the month of Dhul Hijjah .

This hadith, narrated by Umm Salama, wife of Allah's Apostle, encourages Muslims to refrain from cutting their hair and trimming their nails during the month of Dhul Hijjah. This is because it is a time for sacrifice and devotion to Allah. The act of sacrificing an animal is a sign that one has devoted themselves completely to God and has given up something precious in order to please Him. Therefore, it is important for Muslims to show this devotion by refraining from activities such as cutting their hair or trimming their nails until they have completed the sacrifice. By doing so, they are showing that they are willing to give up even small pleasures in order to please God.