D.C current

srinudamawhy internal resistance of a source is connected in series

Because the entire external load current has to pass through the internal resistance to complete the path.

Bioramani

The internal resistance of a source is connected in series because the electricity has to flow from the negative side of the source, through the source and back onto the positive side. For most circuits this internal resistance is so small it is negligible and does not have to be taken into consideration. But again, this depends on your application.

Everett

The internal resistance of a voltage source is connected in series, due to this internal resistor the o/p voltage is less than its said value (like 5 Volt battery). and if you connect a resistor in series with a source there is some voltage drop at at other end of resistor if the circuit is complete. that is why in the practical model of voltage source the internal resistor is connected in series with it.