Muwatta Malik Book 8, Hadith 12
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 Yahya ibn Said that a man asked Said ibn al-Musayyab, "I pray in my house, and then I come to the mosque and find the imam praying. Should I pray with him?" Said said, "Yes," and the man said, "Which of them is my prayer?" Said said, "Are you the one to decide that? That is up to Allah."
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."