Nedarim 8A

מנין שנשבעין לקיים את המצוה, שנאמר: ״נשבעתי ואקימה לשמר משפטי צדקך״.

From where is it derived that one may take an oath to fulfill a mitzva? It is as it is stated: “I have sworn and I have confirmed it, to observe Your righteous ordinances” (Psalms 119:106).

והלא מושבע ועומד מהר סיני הוא. אלא הא קא משמע לן דשרי ליה לאיניש לזרוזי נפשיה.

The Talmud asks: Is he not already under oath from when each Jew took an oath at Mount Sinai to fulfill all the mitzvot? An oath cannot take effect if one is already bound by a different oath. Rather, it teaches us this: It is permitted for a man to motivate himself to fulfill the mitzvot in this manner, although the oath is not technically valid.

ואמר רב גידל אמר רב: האומר ״אשכים ואשנה פרק זה״ ״אשנה מסכתא זו״⁠ — נדר גדול נדר לאלהי ישראל. והלא מושבע ועומד הוא, ואין שבועה חלה על שבועה! מאי קא משמע לן, דאפילו זרוזי בעלמא ⁠— היינו דרב גידל קמייתא!

And Rav Giddel said that Rav said: One who says: I will rise early and study this chapter, or: I will study this tractate, has taken a great vow to the God of Israel. This clearly indicates that the vow takes effect. The Talmud asks: Is he not already under oath due to the general obligation to study Torah? And an oath does not take effect upon a preexisting oath. What is he teaching us? If one claims that Rav Giddel is teaching that such a vow may be made even merely to motivate oneself to study, this is the same as the first statement of Rav Giddel.

הא קא משמע לן: כיון דאי בעי פטר נפשיה בקרית שמע שחרית וערבית, משום הכי חייל שבועה עליה.

The Talmud answers: It teaches us this: Since if he desires he can exempt himself from the obligation to study Torah with the recitation of Shema in the morning and evening, due to that reason the oath takes effect upon him and he is obligated to study the chapter or tractate that he specified.