Or HaChaim on Genesis 30:2
ויחר אף יעקב
Jacob became agitated...
[because]
ודברי הצדיקים אפילו בסדר זה יעשו רושם
the words of the righteous even when okay [conditionally] make an impression.
footnote in the Artscroll Or HaChaim that references this quote with
Sefer HaChinuch, Mitzvah 231
Even thought we do not have the power to know in which way a curse impacts upon the one cursed, and with what power within speech there is to bring [that impact] upon him, we know more generally that people are concerned about curses - whether Israel or other nations - and say that curses of people, and even curses of commoners, have an impact on the one cursed and attaches malediction and distress to him. And since we know this thing from the mouth of the creatures, we will say that it is from the roots of the commandment that God prevented us from injuring others with our mouths, [just] like he prevented us from injuring them with action.
Makkos 11a
Apropos curses that are realized, Rav Yehuda says that Rav says: With regard to the curse of a Sage, even if it is baseless, i.e., based on a mistaken premise, it nevertheless comes to fruition and affects the object of the curse. From where do we derive this? It is derived from this incident involving Ahithophel. When David dug the drainpipes in preparation for building the Temple, the waters of the depths rose and sought to inundate the world. David said: What is the halakha? Is it permitted to write the sacred name on an earthenware shard and throw it into the depths, so that the water will subside and stand in its place? There was no one who said anything to him. David said: Anyone who knows the answer to this matter and does not say it shall be strangled.
Then Ahithophel raised an a fortiori inference on his own and said: And if in order to make peace between a man and his wife in the case of a sota, when the husband suspects his wife of having committed adultery, the Torah says: My name that was written in sanctity shall be erased on the water, then, in order to establish peace for the whole world in its entirety, is it not all the more so permitted? Ahithophel said to David: It is permitted. David wrote the sacred name on an earthenware shard and cast it into the depths, and the water in the depths subsided and stood in its place.
And even so it is written that during the rebellion of Absalom: “And Ahithophel saw that his counsel was not taken, and he saddled his donkey and he arose and went to his house, to his town, and he commanded his household and strangled himself” (II Samuel 17:23). Although David stipulated that his curse would take effect only if one who knows the answer fails to share it with him, and Ahithophel did not fail to share it with him, the curse was realized.
The Gemara cites a similar statement: Rabbi Abbahu says: With regard to the curse of a Sage, even if it is stated conditionally, it comes to realization. From where do we derive this? It is derived from an incident involving Eli the High Priest, as Eli said to Samuel, after the latter had received a prophetic vision with regard to Eli, that his sons do not follow his path: “Therefore may God do to you, and more also, if you hide any matter from me of all the matters that He spoke unto you” (I Samuel 3:17). And even though it is written immediately thereafter: “And Samuel told him all the matters, and did not hide from him” (I Samuel 3:18), it is written at the time of Samuel’s death: “And his sons did not follow in his ways” (I Samuel 8:3), indicating that God did to Samuel as he prophesied with regard to Eli, and his own sons did not follow his path. Despite the fact that Eli stated the curse conditionally, Samuel was affected by the curse.