He recited quietly at the beginning of his prayer, then someone else joined him; should he complete it reciting quietly or out loud?

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Check below answers in case you are looking for other related questions:
How to pray if not sure imam is traveling or resident
The member of the congregation has to follow his imam with regard to matters that are subject to ijtihaad

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Related Answers:
Ja'mmaa'a Prayer with a man and a woman only
Where should a person stand in relation to the imaam if just the two of them are praying together?
He decided (and formed the intention) to carry on praying on his own because the sound of the imam’s voice was cut off, then the sound was restored, and he found that he had gone ahead of his imam in the prayer. What is the ruling?
How to pray if not sure imam is traveling or resident
The member of the congregation has to follow his imam with regard to matters that are subject to ijtihaad

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