**Ignore this extremely bad advice**, as the ADC will cease to function and the internal circuitry may be destroyed.and AREF pin to Ground
The ADC can be affected by noise in the power supply. If using Vcc as the reference, it is common to decouple Vcc and Vcc with a 10 uH inductor as well as capacitors.
See “noise cancelling techniques”, section 24.6 and Figure 24.9 of the ATmega328 data sheet