It is permissible for the latecomer

Q: A man offered `Isha' (Night) Prayer as an Imam (leader of congregational Prayer), and then joined a group of people offering the same Salah as a Ma'mum (a person being led in Prayer). (Part No. 7; Page No. 406) Is this valid or not?

A: If a person has offered a Fard (obligatory) Salah, whether as an Imam, Ma'mum or alone, and then finds a group of people offering the same Salah, it is better to repeat the same Salah with them again. This will be a Nafilah (supererogatory) Salah, while the first is counted as the Fard. Imam Ahmad, Abu Dawud, Ibn Majah and Al-Tirmidhy (may Allah be merciful to them) narrated from Yazid ibn Al-Aswad (may Allah be pleased with him) that he one day offered the Fajr (Dawn) Prayer along with the Messenger of Allah (peace be upon him). When he (peace be upon him) finished Salah, he found two people who did not offer Salah with the congregation. He called for them, and they were brought trembling before him. He asked, "What prevented you from offering Salah with us?" They replied, "We have already offered Salah at home." He said, "Do not do so. If any of you offers Salah at home and finds that the Imam has not offered Salah yet, they should offer Salah with him; and that will be a Nafilah for them." May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.