How did Iblis whisper to Adam while he was in Paradise, when Iblis had already been expelled from it?

A Muslim should not get preoccupied with the details of stories that the Qur'an has encompassed without any authentic narration from the Prophet, peace be upon him. This includes the details of Iblis's whispering to Adam and Eve. The Holy Qur'an confined itself to mentioning the essence of the incident without elaborating.

The noble verses, such as: ("Then Satan whispered to them to make apparent to them that which was concealed from them of their private parts. He said, 'Your Lord did not forbid you from this tree except that you become angels or become of the immortal.' And he swore to them, 'Indeed, I am to you among the sincere advisors.'") Al-A'raf/20-21, and His saying: ("Then Satan whispered to him; he said, 'O Adam, shall I guide you to the tree of eternity and a kingdom that will not deteriorate?'") Taha/120, have interpreted that the whispering was through direct speech and address.

Everything that has been narrated about Iblis entering Paradise or the manner of his speaking to Adam and Eve belongs to the unseen, which cannot be delved into except with authoritative knowledge or from Isra'iliyyat (Judeo-Christian traditions).

What is important is to believe that the story took place and that Satan is a clear enemy to humankind, and one must be wary of him and seek Allah's help.