Connecting supply line to the ADC directly has other implications such as subjecting the ADC line to higher voltage than 5 V, possibly damaging the pin or the whole MCU.
You can form a voltage divider using two resistors. Now you could accommodate chances of the external power input being higher than 5 V too.
Connecting a small capacitor (100nF) across the ADC input will also smooth out the noise on the line.
What is the reference voltage source of the ADC you are planning? Is there internal bandgap voltage reference? It is more important too
I would still connect two Schottky diodes back to back across the ADC for voltages beyond the operating range of the MCU. This will prevent any mishaps just in case.
Image source: Figure 2 from ADC Input Protection - Texas Instruments Application Report SLAA593