interference pattern can not be obtained when two different sources of same wavelength are used. why?

for interference pattern to bee seen the sources should be coharent . coherent sources are those sources which produce light of zero or constant phase difference, having same amplitude , same wavelength. EVEN IF THE TWO  DIFFERENT SOURCES HAVE SAME WAVELENTGH THEY CANNOT BE CONSIDERED AS COHERANT SOURCES THERE FORE CANNOT PRODUCE INTERFERENCE PATTERN