What should I do if the congregational prayer (Salat al-Jama'ah) is established while I am praying a supererogatory (nafila) prayer: should I interrupt my prayer and join the congregation, or should I complete the supererogatory prayer?

If the prayer has been announced, it is not permissible to start a supererogatory prayer, due to the Prophet's (peace and blessings be upon him) saying: "If the prayer has been announced, then there is no prayer except the obligatory one." As for one who is engaged in a supererogatory prayer, he should cut it off without saying the taslim, unless he is in the second unit (rak'ah); in that case, he should complete it lightly, say the taslim, and then join the congregation.