Surah Al-Baqarah 2:249

فَلَمَّا فَصَلَ طَالُوتُ بِٱلْجُنُودِ قَالَ إِنَّ ٱللَّهَ مُبْتَلِيكُم بِنَهَرٍ فَمَن شَرِبَ مِنْهُ فَلَيْسَ مِنِّى وَمَن لَّمْ يَطْعَمْهُ فَإِنَّهُۥ مِنِّىٓ إِلَّا مَنِ ٱغْتَرَفَ غُرْفَةًۢ بِيَدِهِۦ ۚ فَشَرِبُوا۟ مِنْهُ إِلَّا قَلِيلًا مِّنْهُمْ ۚ فَلَمَّا جَاوَزَهُۥ هُوَ وَٱلَّذِينَ ءَامَنُوا۟ مَعَهُۥ قَالُوا۟ لَا طَاقَةَ لَنَا ٱلْيَوْمَ بِجَالُوتَ وَجُنُودِهِۦ ۚ قَالَ ٱلَّذِينَ يَظُنُّونَ أَنَّهُم مُّلَـٰقُوا۟ ٱللَّهِ كَم مِّن فِئَةٍ قَلِيلَةٍ غَلَبَتْ فِئَةً كَثِيرَةًۢ بِإِذْنِ ٱللَّهِ ۗ وَٱللَّهُ مَعَ ٱلصَّـٰبِرِينَ

And when Saul went forth with the soldiers, he said, "Indeed, Allāh will be testing you with a river. So whoever drinks from it is not of me, and whoever does not taste it is indeed of me, excepting one who takes [from it] in the hollow of his hand." But they drank from it, except a [very] few of them. Then when he had crossed it along with those who believed with him, they said, "There is no power for us today against Goliath and his soldiers." But those who were certain that they would meet Allāh said, "How many a small company has overcome a large company by permission of Allāh. And Allāh is with the patient."

Translation: Saheeh International

Tafsir

And when Saul went forth with the hosts from the Holy House sc. Jerusalem the heat was intense and so they asked him for water; he said ‘God will try test you in order to distinguish the obedient among you from the disobedient with a river between Jordan and Palestine whoever drinks of it of its water is not of me is …

Excerpt from Tafsir al-Jalalayn, commentary on 2:249, via Quran.com. The reader has the full commentary and 9 more tafsirs.