As he is about to die, what does King Arthur request of Sir Bedivere? How does Bedivere comply with this request?

In book 21 of Sir Thomas Malory's epic rendition of Arthurian legend, Le Morte d'Arthur, Arthur's army meets the army of Mordred, his bastard son, on a battlefield. Arthur and Mordred are there to parley, but a misunderstanding causes fighting to commence, and in the fray, Arthur kills Mordred, who mortally wounds Arthur in return.
Arthur's knights, Sir Bedivere and Sir Lucan, manage to get Arthur off the field and "to a little chapel not far from the seaside." Arthur asks Sir Lucan to go back to the battlefield and report to him what is happening there; Sir Lucan returns with news that people are robbing the corpses of the dead, and killing the dying in order to steal from them.

Therefore by my rede, said Sir Lucan, it is best that we bring you to some town.

Arthur agrees, saying, "I would it were so," but he is too weak to walk, and when Sir Lucan and Sir Bedivere try to lift him, Sir Lucan collapses and dies of his own wounds. Arthur knows then that there is no help for him and that he will die soon, so he asks Sir Bedivere to take Excalibur and throw it into the water:

take thou Excalibur, my good sword, and go with it to yonder water side, and when thou comest there I charge thee throw my sword in that water, and come again and tell me what thou there seest.

Sir Bedivere, grieving, agrees to do this, but he cannot bring himself to throw the beautiful sword into the sea:

he said to himself: If I throw this rich sword in the water, thereof shall never come good, but harm and loss. And then Sir Bedivere hid Excalibur under a tree.

He goes back to Arthur and claims to have done as Arthur asked. When Arthur asks what Sir Bedivere saw, Sir Bedivere lies, "I saw nothing but waves and winds." Arthur knows from this statement that Sir Bedivere has not thrown Excalibur into the water, and he orders him to go back outside and actually fulfill Arthur's command this time.
Once again, Sir Bedivere thinks it is "sin and shame to throw away that noble sword," so he hides the sword a second time, and reports back to Arthur, saying he saw "nothing but the waters wap and waves wan." Arthur is not happy, and calls Bedivere a traitor, saying,

now hast thou betrayed me twice. Who would have weened that . . . thou art named a noble knight, and would betray me for the richness of the sword. But now go again lightly, for thy long tarrying putteth me in great jeopardy of my life, for I have taken cold. And but if thou do now as I bid thee, if ever I may see thee, I shall slay thee with mine own hands[.]

Chastened by Arthur's fury, Sir Bedivere goes out for a third time and finally complies with the king's wishes, throwing the sword "as far into the water as he might." When he has done this,

there came an arm and a hand above the water and met it, and caught it, and so shook it thrice and brandished, and then vanished away the hand with the sword in the water.

Bedivere runs to tell Arthur what he saw, and Arthur asks Bedivere to take him quickly to the waterside, where a barge of beautiful weeping women arrives to take Arthur away.