The Quran

وَجَعَلنا مِنهُم أَئِمَّةً يَهدونَ بِأَمرِنا لَمّا صَبَروا ۖ وَكانوا بِآياتِنا يوقِنونَ

Muhammad Asad

and [as] We raised among them leaders who, so long as they bore themselves with patience and had sure faith in Our messages, guided [their people] in accordance with Our behest¹ - [so, too, shall it be with the divine writ revealed unto thee, O Muhammad.]²

I.e., in accordance with the divine ordinances enunciated in and for their time in the Torah: an allusion to the decline of faith, frequently mentioned in the Qur'an, among the children of Israel of later times, and the tendency among many of their leaders and learned men to corrupt the text of the Torah and, thus, to "overlay the truth with falsehood" (see, e.g., 2:42, 75, 79, and the corresponding notes).
This interpolation reflects Zamakhshari's commentary on the above passage, to the effect that the Qur'an is destined to provide guidance and light so long as the coinmunity's religious leaders are patient in adversity and steadfast in their faith: an interpretation which implies that the Qur'an will cease to be of benefit to people who have lost their moral virtues and their faith.