In the wikipedia article for Kazhdan's Property (T), there's an intriguing application:

Similarly, groups with property (T) can be used to construct finite sets of invertible matrices which can efficiently approximate any given invertible matrix, in the sense that every matrix can be approximated, to a high degree of accuracy, by a finite product of matrices in the list or their inverses, so that the number of matrices needed is proportional to the number of significant digits in the approximation.

Unfortunately, there's no citation for this and googling invertible matrices + property (T) doesn't give much, nor have I found it in the book by Bekka, de la Harpe, and Valette. Does anyone have a reference for the above quote?

Discrete groups, expanding graphs, and invariant measures". – YCor Jan 22 at 17:02