They said, "O Moses, indeed we will not enter it, ever, as long as they are within it; so go, you and your Lord, and fight. Indeed, we are remaining right here."
Musa Reminds His People of Allah's Favors on Them; The Jews Refuse to Enter the Holy Land
Allah states that His servant, Messenger, to whom He spoke directly, Musa, the son of `Imran, reminded his people that among the favors Allah granted them, is that He will give them all of the good of this life and the Hereafter, if they remain on the righteous and straight path. Allah said,
(And (remember) when Musa said to his people: "O my people! Remember the favor of Allah to you, when He made Prophets among you,) for whenever a Prophet died, another rose among them, from the time of their father Ibrahim and thereafter. There were many Prophets among the Children of Israel calling to Allah and warning against His torment, until `Isa was sent as the final Prophet from the Children of Israel. Allah then sent down the revelation to the Final Prophet and Messenger, Muhammad ﷺ, the son of `Abdullah, from the offspring of Isma`il, the son of Ibrahim, peace be upon them. Muhammad is the most honorable Prophet of all times. Allah said next,
But the Bani Isra'il who had not listened to their own prophet Sayyidna Musa (علیہ السلام) would have hardly listened to these elders. They gave them the same response, even in a manner which was more uncouth and grotesque. They said: فَاذْهَبْ أَنتَ وَرَبُّكَ فَقَاتِلَا إِنَّا هَاهُنَا قَاعِدُونَ (So go, you and your Lord, and fight. As for us, we are sitting right here). Had this remark of the Bani Isra'il been by way of mockery, it would have been open blasphemy (Kufr). After that, the fact of Sayyidna Musa (علیہ السلام) living with them and praying for them in the wilderness of Tih which finds mention in the next verse would have not been possible.
Therefore, the sense of this statement given by leading commentators is: 'You go and fight them. Your Lord will help you. As for us, we cannot help you.' Given this sense, their statement cannot be taken as blasphemy, though the response given by them remains grotesque and hurtful. This is why this statement of the Bani Isra'il became proverbial.
Let us recollect a related episode from the early period of Islam. This is the Battle of Badr. An army contingent of a thousand strong young men starts marching against Muslims, who are hungry and almost unarmed. Seeing this happen, the Holy Prophet ﷺ started praying before his Lord. Then, a Sahabi, Sayyidna Miqdad ibn al-Aswad ؓ stepped forward and said: 'Ya Rasul Allah , we swear by Allah we shall never say what was said to Sayyidna Musa (علیہ السلام) by his people, that is فَاذْهَبْ أَنتَ وَرَبُّكَ فَقَاتِلَا إِنَّا هَاهُنَا قَاعِدُونَ (So go, you and your Lord, and fight. As for us, we are sitting right here). Instead of that, we shall defend you from your right and left and from the front and rear. Please feel free of any concern and go ahead with your battle plan.'
When he heard this, the Holy Prophet I ﷺ was very pleased, and his Companions ؓ too were fired with a renewed zeal for Jihad. Sayyidna ` Abdullah ibn Masud رضی اللہ تعالیٰ عنہما always used to say: 'I really envy this feat of Miqdad ibn al-Aswad. I wish I too had this good fortune.'
In gist, the people of Sayyidna ibn Musa (علیہ السلام) ، bygiving him a flat response at such a crucial time, broke all pledges they had made.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 24–25
It is the way of God that He chooses a group in order to convey His message to the people. He sends His prophet and His Book to this group and assigns them the task of conveying this message to others. Just as a particular person is the recipient of revelation, similarly a particular group is made the bearer of the revelation. In ancient times this special position was enjoyed by the Children of Israel and after the advent of the last of the prophets, his followers (ummah) were appointed to this position. Just as God requires of a community that it should represent His religion, similarly He also wants the community representing His religion to enjoy honours and high position in this world, so that it should be demonstrated to the people that in the new world coming into existence after Doomsday, prominence and distinction will be conferred only upon the ‘People of the Truth’. All others will be subdued and distanced from God’s blessings. However, this worldly benefit is not conferred on this group unconditionally: the group has first to face the test of entitlement. It has to prove that it has full faith in God under all circumstances and bows to His will and pleasure even if that tries their patience to the very limits.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.