Musnad Ahmad 2:108

Waki’ narrated to us, Sufyan narrated to us, from ‘Abd al-Rahman ibn al-Harith ibn ‘Ayyash ibn Abi Rabi’ah, from Hakim ibn Hakim ibn ‘Abbad ibn Hunayf, from Abu Umamah ibn Sahl ibn Hunayf: That a man shot a man with an arrow and killed him, and he had no heir except a maternal uncle. Abu ‘Ubaydah ibn al-Jarrah wrote to ‘Umar regarding that, and he wrote: That the Prophet, peace and blessings of Allah be upon him, said: "Allah and His Messenger are the guardian of the one who has no guardian, and the maternal uncle is the heir of the one who has no heir."