Musnad Ahmad 32:887

Sahl narrated to us, from Humayd, and Abdullah ibn Bakr, narrated to us Humayd, from Thabit, from Anas: "That the Prophet, may Allah bless him and grant him peace, used to sacrifice two white-and-black rams." Ibn Bakr said: "The Prophet, may Allah bless him and grant him peace, sacrificed two white-and-black rams." Both of them attributed it to Thabit, from Anas.