Surah Al Imran Tafseer

وَقَالَت طَّأيِفَةٌ مِّنْ أَهْلِ الْكِتَابِ امِنُواْ بِالَّذِيَ أُنزِلَ عَلَى الَّذِينَ امَنُواْ وَجْهَ النَّهَارِ وَاكْفُرُواْ اخِرَهُ

And a party of the People of the Scripture say:"Believe in the morning in that which is revealed to the believers, and reject it at the end of the day,

This is a wicked plan from the People of the Book to deceive Muslims who are weak in the religion. They decided that they would pretend to be believers in the beginning of the day, by attending the dawn prayer with the Muslims. However, when the day ended, they would revert to their old religion so that the ignorant people would say, "They reverted to their old religion because they uncovered some shortcomings in the Islamic religion."

This is why they said next.

لَعَلَّهُمْ يَرْجِعُونَ

so that they may turn back.

Ibn Abi Najih said that Mujahid commented about this Ayah, which refers to the Jews,

"They attended the dawn prayer with the Prophet and disbelieved in the end of the day in order to misguide the people. This way, people would think that they have uncovered shortcomings in the religion that they briefly followed.