Surah Taha Tafseer

فَتَنَازَعُوا أَمْرَهُم بَيْنَهُمْ

"...And surely, he who invents a lie will fail miserably." Then they debated one with another what they must do,

It has been said that this means that they argued among themselves. So one of them said, "This is not the speech of a magician, but it is the speech of a Prophet."

Another said, "No, he is only a magician."

There are other opinions that have been mentioned about what they discussed. And Allah knows best.

Allah's statement,

وَأَسَرُّوا النَّجْوَى

and they kept their talk secret.

means, they held secret counsel among themselves about this matter