Maliks Muwatta Book 8, Hadith Number 16.
Section : Praying in a Group of People.
Yahya related to me from Malik from Yahya ibn Said that a man used to lead the people in prayer in al-Aqiq (a place near Madina), and Umar ibn Abd al-Aziz sent a message to him forbidding him to do so.
Malik said, “He only forbade him because his father was not known.”