Electro Optical (EO) sampling is a promising nondestructive method for measuring ultra short (sub-ps) electron bunches. The FEMTO slicing experiment at the Swiss Light Source modulates about 3 pC of the 5 nC electron bunch longitudinally. The coherent synchrotron radiation (CSR) emitted by this substructure was sampled by 100 fs long pulses from an Yb fiber laser in EO crystals of gallium phosphide. The broadening of this ps long structure over four turns of the synchrotron could be measured with sub-ps resolution.