Muwatta Malik Book 8, Hadith 11
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 a man asked Abdullah ibn Umar, "Sometimes I pray in my house, and then catch the prayer with the imam. Should I pray with him?" Abdullah ibn Umar said to him, "Yes," and the man said, "Which of them do I make my prayer?" Abdullah ibn Umar said, "Is that up to you? It is up to Allah. He will decide on whichever of them He wishes."
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."