Euclid, Elements
Thomas L. Heath, Sir Thomas Little Heath, Ed.

For, if H, G, K, L are not the least numbers continuously
proportional in the ratios of A to B, of C to D, and of E to F, let them be N, O, M, P.

Then since, as A is to B, so is N to O, while A, B are least, and the least numbers measure those which have the same
ratio the same number of times, the greater the greater and the less the less, that is, the antecedent the antecedent and the consequent the consequent; therefore B measures O. [VII. 20]

For the same reason
C also measures O; therefore B, C measure O; therefore the least number measured by B, C will also measure O. [VII. 35]

But G is the least number measured by B, C;
therefore G measures O, the greater the less: which is impossible.

Therefore there will be no numbers less than H, G, K, L which are continuously in the ratio of A to B, of C to D, and of E to F.

Next, let E not measure K.

Let M, the least number measured by E, K, be taken.

And, as many times as K measures M, so many times let H, G measure N, O respectively, and, as many times as E measures M, so many times let F
measure P also.

Since H measures N the same number of times that G measures O, therefore, as H is to G, so is N to O. [VII. 13 and Def. 20]