What is a cell assembly?
In Principles of Neural Science Eric Kandel, and colleagues wrote:
After this strengthening has occurred, a group of three neurons that are strongly coupled by excitatory synapses form a cell assembly (Figure E–5C). Neuroscientists generally use this term rather imprecisely. One must look to mathematical models of networks for more precise definitions, which generally have some- thing to do with the presence of strong mutual excita- tory interactions within a group of neurons. The word “assembly” emphasizes that the group did not initially exist but was constructed through the strengthening of the synapses of the neurons in the group, which in turn was caused by the simultaneous activation of the neurons.
In  the cell assembly is defined as:
A large group of cells that tend to be active at the same time because they have been activated simultaneously or in close succession in the past.
Sebastian Seung in  defines the cell assembly like this:
... the neurons are excitatory and are mutually connected by synapses into a structure known as a cell assembly.
Hence, my question;
What is the best evidence, if any, for the cell assembly?
: Breedlove, Watson, Rosenberg, Biological Psychology 6th Edition
: Seung, Connectome - How the Brain's Wiring Makes Us Who We Are