The Quran

129

قالوا أوذينا مِن قَبلِ أَن تَأتِيَنا وَمِن بَعدِ ما جِئتَنا ۚ قالَ عَسىٰ رَبُّكُم أَن يُهلِكَ عَدُوَّكُم وَيَستَخلِفَكُم فِي الأَرضِ فَيَنظُرَ كَيفَ تَعمَلونَ

Muhammad Asad

[But the children of Israel] said: "We have suffered hurt ere thou camest to us and since thou hast come to us!¹ [Moses] replied: "It may well be that your Sustainer will destroy your foe and make you inherit the earth: and thereupon he will behold how you act."²

This is, in the above context, the first hint of the inconstancy and weakness of faith for which the Qur'an so often blames the children of Israel: and this, together with what follows in verses 138-140 and 148 ff., is the reason why the story of Moses has been included here among the stories of the earlier prophets whose warnings were neglected by their communities.
I.e., "He will judge you by your actions". As is evident from the reference, in verse 137 below, to the "patience in adversity" which the children of Israel subsequently displayed, it would seem that the hope held out to them by Moses helped them, once again, to overcome their moral weakness; but, at the same time, his words "God will behold how you act" imply a distinct warning.