Muwatta Malik Book 8, Hadith 16
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."
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 Ibn Shihab from Anas ibn Malik that the Messenger of Allah, may Allah bless him and grant him peace, fell off his horse when riding, and his right side was scraped, so he did one of the prayers sitting, and we prayed behind him sitting. When he left, he said, "The imam is appointed to be followed. If he prays standing, then pray standing, and when he goes into ruku, go into ruku, and when he rises, rise, and when he says, 'Allah hears whoever praises him,' say, 'Our Lord, praise belongs to You,' and if he prays sitting, then all of you pray sitting."