Question: Is it allowed to say the Inshallah [الاستثناء ] in regards to Ihsaan and likewise Islam? Or is that specifically for Imaan?
Answer: “It has been reported for Imaan. What is well-known about Ihsan is that it is above Imaan; however, it is used for Ihsaan with wealth. Ihsan is not only for actions alone in what is between him and Allah. It encompasses what is established in the heart and what is established with the limbs. What has come from the Salaf is saying Inshallah for Iman“.
[Sharh Sunan Tirmihi no. 283]
Faisal Ibn Abdul Qaadir Ibn Hassan