Ash-Shaykh Abdul-Muhsin al-Abbaad, may Allah preserve him, was asked the following question,
السؤال: إذا غطى جبين المصلي الشعر أو الطاقية، فما حكم هذا الفعل؟ وهل يلزم إلصاق الجبين على الأرض مباشرة دون حائل؟
If the forehead of the person praying becomes covered by his hair or by a koofi, then what is the ruling on this action, and is it necessary that he touches the floor with his forehead directly without any barrier?
الجواب: الجبهة والأنف هي التي توضع على الأرض، والإنسان عليه ألا يغطي جبهته، لكن إذا كانت هناك عمامة أو طاقية غطت بعض الرأس، أو بعض الجبهة، وسجد على الجبهة، وعلى بعض العمامة أو بعض الطاقية فإن ذلك لا يؤثر.
The forehead and the nose is that which should be placed on the floor. The person shouldn’t cover his forehead. However if there is an imamah or a koofi which has covered some of his head or his forehead, and he were to prostate on his forehead along with some of his imamah or along with some of his koofi, then that doesn’t affect (it).
Reference: Sharh Sunan Abu Dawud (086)
AbdulFattaah bin Uthman