Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him)
What can I do to get rid of satanic whisperings (waswasah) at the time of entering the prayer?
You can do this by being aware of the greatness of Allah. Imam Ahmad bin Hasan al-`Attas was of the opinion that it is sufficient for someone entering the prayer to bring to mind Allah’s greatness. You should recite also Surat al-Nas at this time for protection.
According to the Shafi School the intention must be combined with the opening takbir. People sometimes find this difficult to act upon and as a result they can be affected by whisperings and doubts. Imam Ahmad bin Hasan al-`Attas thus directed people not to worry about whether the intention was made at the correct time, but rather to focus on the meaning of the opening takbir which is to realise and express Allah’s greatness.