From $ROOTSYS/tutorials/fit/fit2.C

#include "TF2.h"
#include "TH2.h"
#include "TMath.h"

// Fitting a 2-D histogram
// This tutorial illustrates :
//  - how to create a 2-d function
//  - fill a 2-d histogram randomly from this function
//  - fit the histogram
//  - display the fitted function on top of the histogram
// This example can be executed via the interpreter or ACLIC
//   root > .x fit2.C
//   root > .x fit2.C++
//Author: Rene Brun

Double_t g2(Double_t *x, Double_t *par) {
   Double_t r1 = Double_t((x[0]-par[1])/par[2]);
   Double_t r2 = Double_t((x[1]-par[3])/par[4]);
   return par[0]*TMath::Exp(-0.5*(r1*r1+r2*r2));
Double_t fun2(Double_t *x, Double_t *par) {
   Double_t *p1 = &par[0];
   Double_t *p2 = &par[5];
   Double_t *p3 = &par[10];
   Double_t result = g2(x,p1) + g2(x,p2) + g2(x,p3);
   return result;

void fit2() {
   const Int_t npar = 15;
   Double_t f2params[npar] =
   TF2 *f2 = new TF2("f2",fun2,-10,10,-10,10, npar);

   //Create an histogram and fill it randomly with f2
   TH2F *h2 = new TH2F("h2","from f2",40,-10,10,40,-10,10);
   Int_t nentries = 100000;
   //Fit h2 with original function f2
   Float_t ratio = 4*nentries/100000;
   f2params[ 0] *= ratio;
   f2params[ 5] *= ratio;
   f2params[10] *= ratio;
   f2->Draw("cont1 same");