Muwatta Malik Book 8, Hadith 15
Yahya related to me from Malik that Nafi said, "I stood behind
Abdullah ibn Umar in one of the prayers when there was no one else
with him, and he reached behind with his hand and placed me beside
him."
Yahya related to me from Malik from Abu'z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "When you lead people in the prayer, make it short, because there are among them some people who are weak, ill and old. But when you pray on your own, make it as long as you wish."
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."