I am reading the textbook algorithms by S. Dasgupta, C.H. Papadimitriou, and U.V. Vazirani
The authors state in page $67$:
The preceding formula implies an $O(n^3)$ algorithm for matrix multiplication: there are $n^2$ entries to be computed, and each takes $O(n)$ time. For quite a while, this was widely believed to be the best running time possible, and it was even proved that in certain models of computation no algorithm could do better. It was therefore a source of great excitement when in $1969$, the German mathematician Volker Strassen announced a significantly more efficient algorithm, based upon divide-and-conquer.
The statement that interests my more in this quotation, is:
it was even proved that in certain models of computation no algorithm could do better.
Could anyone support me with some references where I can find a proof of that kind?