Nafi' bin Jubayr bin Mut'im narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The days of Tashriq (that is, the 11th, 12th and 13th of Dhul-Hijjah) are all days of sacrifice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19242