In the beginning of Dune, I'm afraid I don't remember the exact quote, Paul and Leto are talking about Arrakis. Paul gives a warning to Leto, but remarks internally on "the effort" it takes to say it. It is implied that Paul perceives this to be due to Mohiam's influence on him. Did she actually do anything, or was it just an impression that Paul had that was so strong that he considered himself to be under a spell of some kind that wasn't really there?
It's unclear if the Reverend Mother explicitly laid a compulsion on Paul, but he feels and acts as though she had. Recall that this conversation with her was immediately following his ordeal, during which she used the Voice on him at least twice:
"You think I could be this Kwisatz Haderach," he said. "You talk about me, but you haven't said one thing about what we can do to help my father. I've heard you talking to my mother. You talk as though my father were dead. Well, he isn't!"
"If there were a thing to be done for him, we'd have done it," the old woman growled. "We may be able to salvage you. Doubtful, but possible. But for your father, nothing. When you've learned to accept that as a fact, you've learned a real Bene Gesserit lesson."
So there's no concrete description of a specific threat against his father, just a Bene Gesserit foreknowing that Leto is doomed (and probably Paul too). So there's not a lot for Paul to convey as a warning, just that the Reverend Mother believes Leto will die.
It is immediately after this comment that the Reverend Mother takes her leave, and there's nothing in the text to indicate that she sealed Paul's lips in any way.
But when Paul is talking to Thufir Hawat, he finds it difficult to relate the story of that warning:
"She... " Paul hesitated, found that he couldn't tell Hawat about the ordeal. The inhibitions went deep.
"Yes? What did she?"
Paul took two deep breaths. "She said a thing." He closed his eyes, calling up the words, and when he spoke his voice unconsciously took on some of the old woman's tone: " 'You, Paul Atreides, descendant of kings, son of a Duke, you must learn to rule. It's something none of your ancestors learned.' " Paul opened his eyes, said: "That made me angry and I said my father rules an entire planet. And she said, 'He's losing it.' And I said my father was getting a richer planet. And she said. 'He'll lose that one, too.'
Which suggests that the compulsion is bound more to the ordeal, and keeping it secret, than to the conversation following. Nevertheless Paul still finds it difficult to discuss this with his father:
Paul nodded, suddenly overcome by memory of the Reverend Mother's words: "...for the father, nothing."
"Father," Paul said, "will Arrakis be as dangerous as everyone says?"
The Duke forced himself to the casual gesture, sat down on a corner of the table, smiled. A whole pattern of conversation welled up in his mind -- the kind of thing he might use to dispel the vapors in his men before a battle. The pattern froze before it could be vocalized, confronted by the single thought: This is my son.
"It'll be dangerous," he admitted.
"Hawat tells me we have a plan for the Fremen," Paul said. And he wondered: Why don't I tell him what that old woman said? How did she seal my tongue?
The Duke noted his son's distress, said: "As always, Hawat sees the main chance. But there's much more. I see also the Combine Honnete Ober Advancer Mercantiles -- the CHOAM Company. By giving me Arrakis, His Majesty is forced to give us a CHOAM directorship... a subtle gain."
"CHOAM controls the spice," Paul said.
"And Arrakis with its spice is our avenue into CHOAM," the Duke said. "There's more to CHOAM than melange."
"Did the Reverend Mother warn you?" Paul blurted. He clenched his fists, feeling his palms slippery with perspiration. The effort it had taken to ask that question.
So even though it's not apparent that Reverend Mother Mohiam actually did anything to Paul, he is acting as though he had. It's possible that the compulsion not to reveal anything about the ordeal (which is real) is making it hard for Paul to talk about anything discussed at that time, but it's not clear.