Musnad Abu Ya'la al-Mawsili 155:206

Abu Hisham al-Rifa'i narrated to us, Abu Khalid narrated to us, from Hisham, from Muhammad, from Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), who said: "What do you consider the *raqūb* (childless person) among you to be?" They said: "The one who has no children." He said: "Rather, he is the one who has no *faraṭ* (preceding child who has died)."