Abu al-Asad al-Sulami narrates from his father, from his grandfather (may Allah be pleased with him): I was the seventh with the Messenger of Allah (peace and blessings be upon him). Each one contributed a dirham, and we bought a sacrifice for seven dirhams, and he (peace and blessings be upon him) ordered that the sacrificial animal be held down. Baqiyyah said: I asked Hammad bin Zayd: "Who was the seventh?" He said: I do not know. I said: "It was the Messenger of Allah (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19051