If the prayer is called for and a person is praying the greeting of the mosque or the pre-prayer sunnah, should he break it off or complete it briefly?

Question:
If a person is praying two rak'ahs of sunnah, or the greeting of the mosque, before any prayer, and then the mu'adhdhin stands up and calls for the prayer, should the person break off the prayer or complete it briefly and join the congregation?

Answer:
He should complete it briefly and join the congregation, unless he is proficient in the congregational prayer, in which case he should cut it short. If he is not proficient in the congregational prayer, he should complete it briefly. As for the saying of Allah the Exalted: And as for when the prayer is established and he has not entered into a voluntary prayer, it is not permissible for him to enter into a voluntary prayer, based on the saying of the Messenger (peace and blessings be upon him): This means that it is prohibited for the one who begins a voluntary prayer after the iqamah, and he is forbidden from doing so. As for the one for whom the prayer is established while he is in it, he should complete it briefly, unless he fears missing the Friday prayer, in which case he should cut it short, as the scholars (may Allah have mercy on them) explicitly stated. May Allah reward you with good.