If you have prayed the obligatory prayer at home, then went to the mosque and found the congregation, do you pray with them a supererogatory prayer with a taslim after two rak'ahs in the Maghrib and Isha prayers, or do you complete the prayer with the congregation three rak'ahs in Maghrib and four rak'ahs in Isha?

Men must perform congregational prayers in the mosque unless there is a valid excuse. If a man prays in his home or while traveling, then comes to the mosque and finds the congregation praying, he should pray with them, and it will be a supererogatory prayer for him. The meaning of supererogatory here is that he has already performed his initial prayer, fulfilling his obligatory duty, and his second prayer is a voluntary act. It is prescribed for him to pray with the Imam's prayer, whether it consists of two, three, or four rak'ahs, due to the general command of the Prophet, peace be upon him, to pray with the congregation. This is supported by the hadith of Abu Sa'id al-Khudri, in which the Prophet, peace be upon him, said: "Who will give charity to this man by praying with him?" And the hadith of Mu'adh, who used to pray the obligatory prayer with the Prophet and then lead his people in prayer as a supererogatory act, even for the Maghrib prayer. Therefore, it is not prescribed to separate from the Imam; rather, he should complete his prayer and make the taslim with him.