This is not part of the authentic hadeeth! The last verse of the Surah pauses a question which is a statement actually. The question is: Isn’t Allah the best of judges? The weak narration says that worshippers answer this by saying: Yes, and I testify to that!

I witnessed an event where the imam leads the prayer and recite surah teen and by the end of the last verse of surah teen the followers say in chorus BALLAA. Wat does it mean and is it part of Islam to say it whenever the last verse of surah teen completed?