What is Macbeth's lie to Banquo about the witches' predictions in Act 2 scene 1?

Banquo obviously did dream of the Weird Sisters. That is why he says, either to himself or to his son Fleance:

A heavy summons lies like lead upon me,
And yet I would not sleep. Merciful powers,
Restrain in me the cursed thoughts that nature
Gives way to in repose!

He is afraid to go to sleep again because he feels sure he will have more dreams about the Weird Sisters and what they have promised him. He and Macbeth are both suffering from insomnia because of the Weird Sisters. When he tells Macbeth

I dreamt last night of the three weird sisters:
To you they have show'd some truth

he may be hoping they can commiserate. The three witches have caused both of them serious emotional problems by their promises. But Macbeth has already made concrete plans to fulfill the witches' prophecies as far as they concern him. He doesn't want to discuss them with Banquo for fear he might give himself away. His statement, "I think not of them," is intended to dismiss them as if he thinks their prophecies were nothing but a lot of crazy poppycock and that the fact that they did correctly predict his appointment as Thane of Cawdor was just a coincidence.

But Macbeth has second thoughts. He half-suspects that Banquo might be hinting that he would like to discuss how the two of them might make the prophecies come true. Macbeth could certainly use some help. If he kills Duncan, he still has Malcolm and Donalbain to deal with. Malcolm is Duncan's heir apparent. This may be Macbeth's only opportunity to kill the sons along with their father--but he doesn't know how he can handle three murders all by himself. What he means when he says

Yet, when we can entreat an hour to serve,
We would spend it in some words upon that business,
If you would grant the time

is that he would like to talk to Banquo about the two of them killing Duncan, Malcolm, and Donalbain together. There will never be another opportunity like tonight. The implication seems to be that Macbeth would become king and would then arrange to have Banquo's son his heir apparent. But Banquo doesn't trust Macbeth. If Macbeth is capable of killing Duncan, then he is certainly capable of killing Banquo. And this, in fact, is what Macbeth actually does.

Banquo turns him down. Macbeth has to go it alone. And he botches the job. If he had intended to kill Malcolm and Donalbain, he is thwarted by the voice he imagines crying "Sleep no more!" and then the knocking at the gate which seems to threaten to wake everybody in the castle.