Musnad Ahmad 30:1171

Abu Amir narrated to us, Ali—meaning Ibn al-Mubarak—narrated to us, from Yahya—meaning Ibn Abi Kathir—from Ibn Ya'qub, who said: I heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The *mufarridun* have preceded." They said: O Messenger of Allah, and who are the *mufarridun*? He said: "Those who are obsessed with the remembrance of Allah."