Sayyiduna Mikhnaf bin Sulaym al-Ghamidi (may Allah be pleased with him) said: We were standing with the Prophet (peace and blessings be upon him) at Arafat, and he (peace and blessings be upon him) said: "O people, upon every household each year there is a sacrifice and an atirah. Do you know what the atirah is? It is the animal that you slaughter in the month of Rajab."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19345