Can't add Fit Parameter in Add Entry's Legend

I’m trying to add the output of fit in TLegend, but I have a problem to concatenate char with double.
I tried to do this:

leg->AddEntry(fit1, "2960 mV #mu = " << (const char [15])fit1->GetParameter(0), "l");

But I got:

error: C-style cast from 'Double_t' (aka 'double') to 'const char [15]' is not allowed

Any ideas? I tried to find similar topics but I didn’t get anything useful.

TString::Format("2960 mV #mu = %g", fit1->GetParameter(0))

If I want to limit the precision of GetParameter at the third decimal?