You control at least two other additional creatures and at least three lands.
An additional creature in your graveyard or on the battlefield under your control.
Whisper does not have summoning sickness.
Steps
Activate Whisper by tapping it and sacrificing two other creatures, returning Avenger of Zendikar from your graveyard to the battlefield.
When Avenger of Zendikar enters the battlefield, it triggers itself and Intruder Alarm.
Resolve the Intruder Alarm trigger, untapping all creatures.
Resolve the Avenger of Zendikar trigger, creating at least three 0/1 Plant creature tokens.
Activate Whisper by tapping it and sacrificing Avenger of Zendikar and a Plant token, returning any creature card from your graveyard to the battlefield.
When that creature enters the battlefield, Intruder Alarm triggers.
Resolve the Intruder Alarm trigger, untapping all creatures.