Muwatta Malik Book 8, Hadith 14
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 Nafi that Abdullah ibn Umar used to say, "Someone who prays maghrib or subh and then catches them with the imam should not repeat them."
Malik said, "I do not see any harm in someone who has already prayed in his house praying with the imam, except for maghrib, because if he repeats it, he makes it even."
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."