The Sunnah indicates that the one who wants to offer a sacrifice must stop cutting his hair and nails and removing anything from his skin, from the beginning of the ten days until after he has offered his sacrifice, because the Prophet ﷺ said:
سَمِعْتُ سَعِيدَ بْنَ الْمُسَيَّبِ، يَقُولُ سَمِعْتُ أُمَّ سَلَمَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم تَقُولُ قَالَ رَسُولُ اللَّهِ
صلى الله عليه وسلم‏‏ مَنْ كَانَ لَهُ ذِبْحٌ يَذْبَحُهُ فَإِذَا أُهِلَّ هِلاَلُ ذِي الْحِجَّةِ فَلاَ يَأْخُذَنَّ مِنْ شَعْرِهِ وَلاَ مِنْ

أَظْفَارِهِ شَيْئًا حَتَّى يُضَحِّيَ ‏

Umm Salama, the wife of Allah Apostle ﷺ, reported Allah Messenger ﷺ to have said: He who has a sacrificial animal with him whom (he intends) to offer as sacrifice, and he enters the month of DhuI-Hijjah, he should not get his hair cut or nails trimmed until he has sacrificed the animal.

According to another report, the Prophet ﷺ said:
إِذَا دَخَلَتِ الْعَشْرُ وَأَرَادَ أَحَدُكُمْ أَنْ يُضَحِّيَ فَلاَ يَمَسَّ مِنْ شَعَرِهِ وَبَشَرِهِ شَيْئًا

When any one of you intending to sacrifice the animal enters in the month (of Dhul-Hijjah) he should not get his hair or nails touched (cut). [reported by Muslim with various isnads]

The Prophet ﷺ’s instruction here makes one thing obligatory and his prohibition makes another haram, according to the soundest opinion, because these commands and prohibitions are unconditional and unavoidable. However, if a person does any of these things deliberately, he must seek Allah’s forgiveness but is not required to offer (an extra) sacrifice in expiation; his sacrifice will be acceptable. Whoever needs to remove some hair, nails, etc, because it is harming him, such as having a broken nail or a wound in a site where there is hair, should do so, and there is nothing wrong with that. The state of ihram is so important that it is permitted to cut one’s hair if leaving it will cause harm. There is nothing wrong with men or women washing their heads during the first ten days of Dhul-Hijjah, because the Prophet (peace and blessings of Allah be upon him) only forbade cutting the hair, not washing it.

The wisdom behind this prohibition of the one who wants to offer a sacrifice from cutting his hair etc., is so that he may resemble those in ihram in some aspects of the rites performed, and so that he may draw closer to Allah by offering the sacrifice. So he leaves his hair and nails alone until the time when he has offered his sacrifice, in the hope that Allah will save him in his entirety from the Fire.

If a person has cut his hair or nails during the first ten days of Dhul-Hijjah because he was not planning to offer a sacrifice, then he decides later, during the ten days, to offer a sacrifice, then he must refrain from cutting his hair and nails from the moment he makes this decision.

Some women may delegate their brothers or sons to make the sacrifice on their behalf, then cut their hair during these ten days. This is not correct, because the ruling applies to the one who is offering the sacrifice, whether or not he (or she) delegates someone else to carry out the actual deed. The prohibition does not apply to the person delegated, only to the person who is making the sacrifice, as is indicated in the hadith. The person who is sacrificing on behalf of someone else, for whatever reason, does not have to adhere to this prohibition.

This prohibition appears to apply only to the one who is offering the sacrifice, not to his wife and children, unless any of them is offering a sacrifice in his or her own right, because the Prophet ﷺ used to sacrifice "on behalf of the family of Muhammad," but there are no reports that say he forbade them to cut their hair or nails at that time.

The things that are described above as being prohibited for the person who is planning to offer a sacrifice are reported in the hadith quoted above; the person is not forbidden to wear perfume, have marital relations, wear sewn garments, etc.

The Takbeeraat of Tashreeq

It is Wajib (incumbent) for every adult Muslim male to recite the Takbeeraat of Tashreeq after every Fardh Salaat, performed with Jamaat or individually, from the Fajr of the 9th of Dhul-Hijjah to the Asr of the 13th of Dhul Hijjah. The Takbeeraat should be recited once only.

The words of the Takbeeraat of Tashreeq are as follows:

اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ لَا إلَهَ إلَّا اللَّهُ وَاَللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ وَلِلَّهِ الْحَمْدُ

“Allah is the Greatest, Allah is the Greatest. There is no deity besides Allah and Allah is the Greatest. Allah is the Greatest and all praises belong to Him Alone.”

NB. Men should recite this Takbeer audibly whilst females should do so softly.