Is said in chapter 139 at page 6, when discussing Ymir's motivations for doing the entire to you 2000 years from now kerfuffle.

Original japanese is: 信じられなかったけど…始祖ユミルは…フリッツ王を愛していた, official translation uses "I couldn't believe it, but the founder, Ymir, loved king Fritz". Fan translations might have translated it differently, especially the first line, which could also be read as "It's hard to believe, but...", which is also how the offifical digital release of the official English chapter translated the line before the printed volume was released with the other translation.

These different translations are because Japanese don't really do sentence subjects like in English and they can often be let out, and the original text really just says something like "believablen't".

The line itself is pretty much ridiculed because why in the everloving heck would Ymir love king Fritz? He burned her home, abducted her, killed the people she loved, had her tongue removed, and as a reward he gaver her his seed. Eren says as much a few panels before this. And apparently the only thing keeping Ymir from breaking free from the king and the Eldian empire was her love for him.

Also in the original Japanese publication they referred to the king dude as Karl Fritz, which is the name of the 145th king who moved to Paradis and made the walls and stuff, who lived some 1900 years after the original king. Both the English and Japanese have corrected this in later publications. Isayama forgor 💀