1,981. Issuing the Warning
Hilchos Sotah 1:4
If a man told his wife in front of two witnesses not to speak to a certain man, this is not considered a warning. Even if she enters into seclusion with him in front of witnesses and remains there long enough to consummate relations, she is not prohibited and she can’t be made to drink the bitter water based on such a warning.
Hilchos Sotah 1:5
Similarly, if a man told his wife not to be secluded with A and witnesses saw her speaking with A, this is not considered seclusion. She is not prohibited and can’t be made to drink the waters. If the woman wasn’t warned at all and two witnesses came and testified that she secluded herself with a man and remained there long enough to have relations, she is not prohibited and cannot be made to drink.