Muwatta Malik Book 8, Hadith 13
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 from Afif as-Sahmi that a man from the tribe of Bani Asad asked Abu Ayyub al-Ansari. "Sometimes I pray in my house, and then come to the mosque and find the imam praying. Should I pray with him?" Abu Ayyub said, "Yes, pray with him, for some one who does so has the reward of the group, or the equivalent of the reward of the group."
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."