Rabbi Meir says that a condition that does not follow the model of the condition made with the tribes of Gad and Reuven is not a valid condition.
On the daf / in context: Kiddushin 61b requires that a conditional obligation state both the positive and the negative clauses. Avraham’s wording to Eliezer supplies an early model of a properly doubled condition.
Type of Learning
Custom (Bavli — form of a valid condition, R’ Meir)
What Is Learned
A binding condition must state both the affirmative and the negative outcomes, following the pattern of the tribes of Gad and Reuven and of Avraham’s release of Eliezer if the woman refuses.