Poincare wrote (english translation of the french) in 1903: ``A very small cause that escapes our notice determines a considerable effect that we cannot fail to see, and then we say that the effect is due to chance. If we knew exactly the laws of nature and the situation of the universe at the initial moment, we could predict exactly the situation of that same universe at a succeeding moment. But even if it were the case that the natural laws had no longer any secret for us, we could still only know the initial situation approximately. If that enabled us to predict the succeeding situation with the same approximation, that is all we require, and we should say that the phenomenon had been predicted, that it is governed by laws. But it is not always so; it may happen that small differences in the initial conditions produce very great ones in the final phenomena. A small error in the former will produce an enormous error in the latter. Prediction becomes impossible, and we have the fortuitous phenomenon.''
Where?* Poincare wrote a lot of small papers, and gave many talks, and traveled.
*Incidentally, Leibniz and Lichtenberg wrote similar things earlier, so this is a not a priority dispute question regarding instability. Later, but before modern citations, there was also, A. Compton, A. Lotka, E. Milne, J. Wheeler (a cigarette smoke causes a hurricane here and not elsewhere, about a year prior E. Lorenz bird and butterfly examples), etc, so definitely not a priority question.