Musnad Ahmad 578:1

Abdullah ibn Yazid narrated to us, he said: Haywah ibn Shurayh narrated to us, Abu Uthman al-Walid ibn Abi al-Walid al-Madani narrated to us, that Imran ibn Abi Anas narrated to him, from Abu Khirash al-Sulami, that he heard the Prophet, peace and blessings of Allah be upon him, saying: "Whoever forsakes his brother for a year, it is as if he has shed his blood."