Come to prayer with calmness and tranquility. Then pray the part you get (long with the imam) and complete afterwards the part you miss.

Abu Dawud said: Ibn Sirin narrated from Abu Hurairah the words: he should complete it afterwards. Similarly, Abu Rafi' narrated from Abu Hurairah and Abu Dharr narrated from him the words then complete it, and complete it afterwards. There is a variation of words in the narration from him.

The Messenger of Allah (ﷺ) saw a person praying alone. He said: Is there any man who may do good with this (man) and pray along with him.

Yazid prayed along with the Messenger of Allah (ﷺ) when he was a young boy. When he (the Prophet) had prayed there were two persons (sitting) in the corner of the mosque; they did not pray (along with the Prophet). He called for them. They were brought trembling (before him). He asked: What prevented you from praying along with us? They replied: We have already prayed in our houses. He said: Do not do so. If any of you prays in his house and finds that the imam has not prayed, he should pray along with him; and that will be a supererogatory prayer for him.

I said the morning prayer along with the prophet (ﷺ) at Mina. He narrated the rest of the tradition to the same effect.

I came while the Prophet (ﷺ) was saying the prayer. I sat down and did not pray along with them. The Messenger of Allah (ﷺ) turned towards us and saw that Yazid was sitting there. He said: Did you not embrace Islam, Yazid? He replied: Why not, Messenger of Allah; I have embraced Islam. He said: What prevented you from saying prayer along with the people? He replied: I have already prayed in my house, and I thought that you had prayed (in congregation). He said: When you come to pray (in the mosque) and find the people praying, then you should pray along with them, though you have already prayed. This will be a supererogatory prayer for you and that will be counted as obligatory.