Reference: Fataawa tata’allaq bi-Jamaa’ah al-Masjid – Question 27, Page 26
Liqaa. al-Baab al-Maftooh – Volume 24, Page 14
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 voluntary [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 [voluntary] prayer [in order to join the congregational prayer].