John Gill

And looking upon Jesus as he walked,.... Either by them; or as he was going from them to his lodgings; it being toward the close of the day, when John had finished his work for that day, and the people were departing home: John fixed his eyes intently on Christ, with great pleasure and delight, and pointing at him, he saith, behold the Lamb of God; as in Joh 1:29, where it is added, "which taketh away the sin of the world"; and which the Ethiopic version subjoins here.