Hanzalah Abu Khaldah (may Allah have mercy on him) said: Sayyiduna Ammar bin Yasir (may Allah be pleased with him) said: "O Hanzalah, the grazing livestock have been made lawful for you, and this was revealed only concerning what the womb conceals: when its formation is complete and its hair has grown, then the slaughter of its mother is its slaughter."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19497