In the Lord of the Rings series Sméagol/Gollum murders his friend Déagol for the ring. It is also stated that Déagol found the ring in Gladden Fields.
How did the ring get there? And which book this is described in?
@Helmar is correct, but here it is from Gandalf's mouth in the Fellowship of the Ring Chapter 2:
‘But the Ring was lost. It fell into the Great River, Anduin, and vanished. For Isildur was marching north along the east banks of the River, and near the Gladden Fields he was waylaid by the Orcs of the Mountains, and almost all his folk were slain. He leaped into the waters, but the Ring slipped from his finger as he swam, and then the Orcs saw him and killed him with arrows.’ Gandalf paused. ‘And there in the dark pools amid the Gladden Fields,’ he said, ‘the Ring passed out of knowledge and legend; and even so much of its history is known now only to a few, and the Council of the Wise could discover no more. But at last I can carry on the story, I think.
Isildur's fault. From lotr wiki:
Isildur put on the Ring though it pained him and headed for the Anduin. He removed his armor and waded into the River intending to try to cross it. The current was strong and despite his great strength, it pulled him toward the marshes of the Gladden Fields. Then the Ring left Isildur's finger and was lost in the waters.
The One Ring remained in the Gladden Fields until Déagol found it in TA 2463.