Musnad Ahmad 2:241

Yahya ibn Adam narrated to us, Sufyan narrated to us, from Abd al-Rahman ibn Ayyash, from Hakim ibn Hakim, from Abu Umamah ibn Sahl, who said: Umar wrote to Abu Ubaydah ibn al-Jarrah: "Teach your boys swimming and your fighters archery." They used to frequent the targets, and a stray arrow came to a boy and killed him. Its origin could not be found, and he was under the care of a maternal uncle. Abu Ubaydah wrote to Umar regarding him: "To whom should I pay his blood money?" Umar wrote back to him: "The Messenger of Allah, peace and blessings of Allah be upon him, used to say: 'Allah and His Messenger are the guardian of him who has no guardian, and the maternal uncle is the heir of him who has no heir.'"