Answered by Shaykh Hamza Karamali, SunniPath Academy Teacher
If one is praying a sunna prayer before a group prayer, and the group prayer starts before one finishes, what should one do?
In the Name of Allah, Most Merciful and Compassionate
The Position of the School
According to the Shafii school, if one is praying a supererogatory prayer and the iqama (the call to commence the group prayer) is called for one of the five obligatory prayers, it is recommended that one complete ones prayer and then join the group prayer after one has finished.
If, however, one does not expect to be able to complete ones supererogatory prayer before the Imam gives salams, then it would be recommended to interrupt ones prayer and join the group, unless one is confident that one will find another group to pray with (in which case one completes ones prayer and then prays with the other group).
All of this assumes that the group prayer is not the Friday prayer. If it is, then one must interrupt ones prayer and join the group if one does not expect to be able to catch the final rakah.
(Bushra al-Karim bi Sharh Masail al-Talim, 1.134; Ianatul-Talibin, 2.15)
What about the hadith?
The Prophet (Allah bless him and give him peace) said, When the call to commence the group prayer (iqama) is given, let there be no prayer except for the obligatory prayer (maktuba). (Muslim). This hadith is often used to adduce that when the iqama is called, one must interrupt whatever prayer one is praying and join the group. The Shafiis, however, understood this hadith to mean that one should not commence another prayer after the iqama; not that one should interrupt a prayer that one has already begun. Imam Nawawi explained in his commentary on the above hadith:
This is an explicit prohibition against commencing a supererogatory (nafila) prayer after the iqama, regardless of whether the prayer is one of the sunnas before and after the obligatory prayers (such as the sunnas of the fajr, zuhr, and asr prayers), or another prayer. This is the position of Shafii and the majority of the fuqaha (jumhur) (al-Minhaj: Sharh Sahih Muslim b. al-Hajjaj, 5-6.228)
And Allah knows best.