Imam al-Shafi'i (may Allah have mercy on him) said: It is possible that he ordered the repetition because the sacrifice is obligatory, and it is also possible that (he meant) he should repeat it if he wished to sacrifice, since a sacrifice before its time does not suffice. The sacrifice is not obligatory in such a way that leaving it is impermissible; rather it is a sunnah whose observance we love and whose abandonment we dislike, without holding it to be obligatory. If it is asked: Where is the sunnah that indicates it is not obligatory?
(b) Umm Salamah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "When the ten days of Dhu al-Hijjah begin, the person who intends to offer a sacrifice should not remove any hair from any part of his body." Imam al-Shafi'i (may Allah have mercy on him) said: This hadith indicates that the sacrifice is not obligatory, because he (peace and blessings be upon him) said: "Whoever intends to offer a sacrifice." Had the sacrifice been obligatory, he (peace and blessings be upon him) would not have said this; rather he would have said: Let no one cut his hair until he has offered the sacrifice.
The Shaykh said: Al-Bara' bin Azib (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) delivered a sermon on the day of sacrifice: "The first thing to do on this day is to offer the prayer, then to sacrifice. Whoever does this has attained our sunnah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19027