Performing Voluntary Prayers When Congregational Prayer Begins

Question: Noble Shaykh, what is the overall opinion as to what to do when performing the naafilah prayer and the (congregational) prayer has started?

Response: The overall opinion regarding the naafilah (prayer) is that if the (congregational) prayer has started while he is in the second (final) rak'ah, then he should briskly complete it. If he is in the first rak'ah, then he should break his (naafilah) prayer (in order to join the congregational prayer).

Shaykh Ibn 'Uthaymeen (rahimahUllaah)

Fataawa tata'allaq bi-Jamaa'ah al-Masjid: Question 27, Page 26

Liqaa. al-Baab al-Maftooh: Volume 24, Page 14