. - Sheikh Bin Baz

Whoever prayed alone then another Person joined Him, is His Prayer correct?

Question:

If I entered the mosque after the congregational prayer had finished and I began to pray, making the opening Takbir, then a man arrived after me and joined me in my prayer, when I had not intended this. Is his prayer correct or not?
Answer:

The correct opinion is that it is lawful for you to make the intention to be the Imam when one or more persons join you in prayer, because the congregation is required and there is great virtue in it. Some scholars held that this is only permissible in the non-obligatory prayers, but the correct view is that it is permissible in obligatory and non-obligatory prayers, because the basic principle is that that they are essentially the same in rulings, except where some evidence proves otherwise, and it has been authentically reported from him (sallallaahu alaihi wa sallam) that he prayed alone at night in the house of Maimunah, the maternal aunt of Ibn Abbas - may Allaah be pleased with all of them - and Ibn Abbas, may Allaah be pleased with him, got up and made Wudhu and stood on his left side, but the Prophet (sallallaahu alaihi wa sallam) moved him until he was on his right side and led him in prayer... [1] Muslim narrated in his Sahih from the Prophet (sallallaahu alaihi wa sallam) that he was praying alone and Jabir and Jabbar (may Allaah be pleased with them both) came and prayed on his right and left sides, but he placed them both behind him and led them in prayer. [2] These two Hadiths prove what we have mentioned, just as they prove that one person should stand on the Imams right, but that two or more stand behind him.
[1] Al-Bukhari nos. 698 and 699. [2] Muslim no. 3010.

Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Vol. 2 Page 177

Islamchoice