“So when Talut departed with the forces, he said: Surely Allah will try you with a river; whoever then drinks from it, he is not of me, and whoever does not taste of it, he is surely of me, except he who takes with his hand as much of it as fills the hand; but with the exception of a few of them they drank from it. So when he had crossed it, he and those who believed with him, they said: We have today no power against Jalut and his forces. Those who were sure that they would meet their Lord said: How often has a small party vanquished a numerous host by Allah’s permission, and Allah is with the patient”
Abu Baseer narrates Imam Jafar Sadiq (as) said, “The companions of Talut were tested by a river and the companions of Imam Mahdi (as) will be tested similarly.” (Al Ghaybah by Nomani page 316)
(The above tafseer is taken from the book Al Muhajjah)