SOLUTION

An ideal current source (also called constant current source) is one that will supply the same current to any resistance connected across its terminals. The ideal current source has infinite resistance.

The internal resistance is the ratio of change of voltage across the device to the change in current through it, we see that the internal resistance must be infinitely large. The voltage across the source changes but the current, in an ideal source, does not. Hence the change in the current is zero, and any number, when divided by zero, gives an infinitely large quotient.