Coherent sources : Two sources of light are said to be coherent if the phase difference between the emitted waves remains constant.
It is not possible to observe interference pattern with light from any two different sources. This is because, no observable interference phenomenon occurs by' superposing light from two different sources. This happens due to the fact that different sources emit waves of different frequencies. Even if the two sources emit light of the same frequency, the phase difference between the wave trains from them fluctuates randomly and rapidly, i.e, they are not coherent.
Consequently, the interference pattern will change randomly and rapidly, and steady interference pattern would not be observed.