C. F. Keil and Franz Delitzsch on 1 Samuel 15:5

1Sa 15:5

He then advanced as far as the city of the Amalekites, the situation of which is altogether unknown, and placed an ambush in the valley. ויּרב does not come from ריב, to fight, i.e., to quarrel, not to give battle, but was understood even by the early translators as a contracted form of ויּארב, the Hiphil of ארב. And modern commentators have generally understood it in the same way; but Olshausen (Hebr. Gramm. p. 572) questions the correctness of the reading, and Thenius proposes to alter בּנּחל ויּרב into מלחמה ויּערך. נחל refers to a valley in the neighbourhood of the city of the Amalekites.