If the congregational prayer is established, is it better for the one praying alone to break off his prayer or to intend the remainder as a voluntary prayer?

Question:
May Allah bless you. Abu 'Abdullah from Tabuk says: I prayed the midday prayer alone in the mosque, and while I was in the final tashahhud, a group entered and prayed alone. Is it better for me to intend the remainder of my prayer as a voluntary prayer, or to break it off and join them?

Answer:
No, do not break it off. Complete it as a voluntary prayer, then join them and perform the obligatory prayer with them. Yes.