Shaikh Abdul Muhsin al-‘Abbad (may Allah preserve him) was asked:
I left my thawb with a tailor in order to shorten it to the middle of the shins. When I wore it, it became above the ankle, is this permissible to wear it?
If the thawb falls below the ankles in the state of Ruku’, then this does not affect it. This occurrence of the thawb being below the ankle is due to the ruku. As long as it (the thawb) in the usual state is within the Legislated distance [i.e. between the shin until the ankle]”
[Sharh Abu Dawood no. 459]
Faisal bin Abdul Qaadir bin Hassan