What is the reason for the prohibition in his saying (peace and blessings be upon him): {But what Allah alone wills}, given that he permitted it with "thumma"?

The student: ...

The Shaykh : Yes, this is what we said, but the reason is that the "waw" is prohibited and is permitted, while he was silent regarding the "fa"... so, as for permissibility, the precautionary approach is... we do not assert prohibition, due to the clear difference between it and the "waw," and we do not assert permissibility, due to the clear difference between it and . So, as a precaution, we say: we should avoid this matter.