From "Manual of Muad'Dib" by the Princess Irulan

There's another thing , Jessica thought. Paul must be cautioned about their women. One of these desert women would not do as wife to a Duke. As concubine, yes, but not as wife .
Then she wondered at herself, thinking: Have I been infected with his schemes? And she saw how well she had been conditioned. I can think of the marital needs of royalty without once weighing my own concubinage. Yet . . . I was more than concubine .
"Mother."
Paul stopped in front of her. Chani stood at his elbow.
"Mother, do you know what they're doing back there?"
Jessica looked at the dark patch of his eyes staring out from the hood. "I think so."
"Chani showed me . . . because I'm supposed to see it and give my . . . permission for the weighing of the water."
Jessica looked at Chani.
"They're recovering Jamis' water," Chani said, and her thin voice came out nasal past the nose plugs. "It's the rule. The flesh belongs to the person, but his water belongs to the tribe . . . except in the combat."
"They say the water's mine," Paul said.
Jessica wondered why this should make her suddenly alert and cautious.
"Combat water belongs to the winner," Chani said. "It's because you have to fight in the open without stillsuits. The winner has to get his water back that he loses while fighting."
"I don't want his water," Paul muttered. He felt that he was a part of many images moving simultaneously in a fragmenting way that was disconcerting to the inner eye. He could not be certain what he would do, but of one thing he was positive: he did not want the water distilled out of Jamis' flesh.
"It's . . . water," Chani said.
Jessica marveled at the way she said it. "Water ." So much meaning in a simple sound. A Bene Gesserit axiom came to Jessica's mind: "Survival is the ability to swim in strange water ." And Jessica thought: Paul and I, we must find the currents and patterns in these strange waters . . . if we're to survive .
"You will accept the water," Jessica said.
She recognized the tone in her voice. She had used that same tone once with Leto, telling her lost Duke that he would accept a large sum offered for his support in a questionable venture—because money maintained power for the Atreides.
On Arrakis, water was money. She saw that clearly.
Paul remained silent, knowing then that he would do as she ordered—not because she ordered it, but because her tone of voice had forced him to re-evaluate. To refuse the water would be to break with accepted Fremen practice.
Presently Paul recalled the words of 467 Kalima in Yueh's O.C. Bible. He said: "From water does all life begin."
Jessica stared at him. Where did he learn that quotation? she asked herself. He hasn't studied the mysteries .
"Thus it is spoken," Chani said. "Giudichar mantene: It is written in the Shah-Nama that water was the first of all things created."
For no reason she could explain (and this bothered her more than the sensation), Jessica suddenly shuddered. She turned away to hide her confusion and was just in time to see the sunset. A violent calamity of color spilled over the sky as the sun dipped beneath the horizon.
"It is time!"
The voice was Stilgar's ringing in the cavern. "Jamis' weapon has been killed. Jamis has been called by Him, by Shai-hulud, who has ordained the phases for the moons that daily wane and—in the end—appear as bent and withered twigs." Stilgar's voice lowered. "Thus it is with Jamis."
Silence fell like a blanket on the cavern.
Jessica saw the gray-shadow movement of Stilgar like a ghost figure within the dark inner reaches. She glanced back at the basin, sensing the coolness.
"The friends of Jamis will approach," Stilgar said.
Men moved behind Jessica, dropping a curtain across the opening. A single glowglobe was lighted overhead far back in the cave. Its yellow glow picked out an inflowing of human figures. Jessica heard the rustling of the robes.
Chani took a step away as though pulled by the light.
Jessica bent close to Paul's ear, speaking in the family code: "Follow their lead; do as they do. It will be a simple ceremony to placate the shade of Jamis."