Sayyidah 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 and one of you wishes to offer a sacrifice, let him not cut the hair from any part of his body." Sufyan was told that some people do not narrate this hadith as marfu' (attributed to the Prophet). He said: But I narrate it as marfu'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19041