https://www.feynmanlectures.caltech.edu/I_05.html#Ch5-S6

Edit: you may be talking about the paragraph "For an atom at rest," in https://www.feynmanlectures.caltech.edu/III_07.html#Ch7-S1

The section starts saying that "We want now to talk a little bit about the behavior of probability amplitudes in time. [...] We are always in the difficulty that we can either treat something in a logically rigorous but quite abstract way, or we can do something which is not at all rigorous but which gives us some idea of a real situation—postponing until later a more careful treatment. With regard to energy dependence, we are going to take the second course. We will make a number of statements. We will not try to be rigorous—but will just be telling you things that have been found out, to give you some feeling for the behavior of amplitudes as a function of time. "