Musnad Ahmad 32:2112

Affan narrated to us, Hammad narrated to us, Thabit and Ali ibn Zayd informed us, from Anas ibn Malik, that when the polytheists overwhelmed the Prophet, peace and blessings of Allah be upon him, while he was with seven of the Ansar and two men from Quraysh, he said: "Who will repel them from us, and he will be my companion in Paradise?" A man from the Ansar came forward and fought until he was killed. When they overwhelmed him again, he said: "Who will repel them from me, and he will be my companion in Paradise?" This continued until the seven were killed. Then the Messenger of Allah, peace and blessings of Allah be upon him, said to his two companions: "We have not treated our brothers fairly."