Imam al-Shafi'i (may Allah have mercy on him) said: If someone asks: What shows that it (refraining from taking from the hair and nails) is a recommended choice and not obligatory? He is told: Sayyidah Aishah (may Allah be pleased with her) said: I used to make the garlands for the sacrificial animals of the Messenger of Allah (peace and blessings be upon him) with my own hands; then the Messenger of Allah (peace and blessings be upon him) would garland them and send the sacrificial animals off with my father, and nothing became forbidden to the Messenger of Allah (peace and blessings be upon him) until the sacrifice was slaughtered.
Imam al-Shafi'i (may Allah have mercy on him) said: In this is evidence for what I have described, and that a person does not enter the state of ihram merely by sending off his sacrificial animal; and sending off a sacrificial animal is weightier than merely intending to offer a sacrifice.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19044