وَإِذَا لَقُواْ الَّذِينَ امَنُواْ قَالُواْ امَنَّا وَإِذَا خَلَ بَعْضُهُمْ إِلَىَ بَعْضٍ
And when they (Jews) meet those who believe (Muslims), they say, "We believe", but when they meet one another in private...,
Muhammad bin Ishaq reported that Ibn `Abbas commented,
وَإِذَا لَقُواْ الَّذِينَ امَنُواْ قَالُواْ امَنَّا
(And when they (Jews) meet those who believe (Muslims), they say, "We believe"),
"They believe that Muhammad is the Messenger of Allah, `But he was only sent for you (Arabs)."'
قَالُواْ أَتُحَدِّثُونَهُم بِمَا فَتَحَ اللّهُ عَلَيْكُمْ لِيُحَأجُّوكُم بِهِ عِندَ رَبِّكُمْ
they say, "Shall you (Jews) tell them (Muslims) what Allah has revealed to you, that they (Muslims) may argue with you (Jews) about it before your Lord."
However, when they meet each other they say, "Do not convey the news about this Prophet to the Arabs, because you used to ask Allah to grant you victory over them when he came, but he was sent to them (not to you)."
Allah then revealed,
وَإِذَا لَقُواْ الَّذِينَ امَنُواْ قَالُواْ امَنَّا وَإِذَا خَلَ بَعْضُهُمْ إِلَىَ بَعْضٍ قَالُواْ أَتُحَدِّثُونَهُم بِمَا فَتَحَ اللّهُ عَلَيْكُمْ لِيُحَأجُّوكُم بِهِ عِندَ رَبِّكُمْ أَفَلَ تَعْقِلُونَ
And when they (Jews) meet those who believe (Muslims), they say, "We believe," but when they meet one another in private, they say, "Shall you (Jews) tell them (Muslims) what Allah has revealed to you, that they (Muslims) may argue with you (Jews) about it before your Lord."
meaning, "If you admit to them that he is a Prophet, knowing that Allah took the covenant from you to follow him, they will know that Muhammad is the Prophet that we were waiting for and whose coming we find foretold of in our Book. Therefore, do not believe in him and deny him."
Al-Hasan Al-Basri said,
"When the Jews met the believers they used to say, `We believe.' When they met each other, some of them would say, `Do not talk to the companions of Muhammad about what Allah has foretold in your Book, so that the news (that Muhammad is the Final Messenger) does not become a proof for them against you with your Lord, and, thus, you will win the dispute."'
أَفَلَ تَعْقِلُونَ
Have you (Jews) then no understanding!
Allah said,
أَوَلَا يَعْلَمُونَ أَنَّ اللّهَ يَعْلَمُ مَا يُسِرُّونَ وَمَا يُعْلِنُونَ