Incubation Druid does not have summoning sickness.
Incubation Druid has at least one +1/+1 counter on it.
Steps
Activate Incubation Druid's first ability by tapping it, adding three mana of any color a land you control could produce.
Leyline of Abundance triggers, adding ({G} magic symbol) .
Activate Incubation Druid by paying ({3} magic symbol) and untapping it, giving itself +2/+2 until end of turn.
Repeat.
Once you have infinite ({G} magic symbol) , you may activate Leyline of Abundance's ability to put infinite +1/+1 counters on each creature you control.
Results
Infinite +1/+1 counters.
Infinitely large Incubation Druid until end of turn.