Sahih al-Bukhari 836
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · No cleaning (rubbing) one's forehead and nose till one has completed As-Salat (the prayer)
Narrated Abu Sa`id Al-Khudri:
حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ ، قَالَ حَدَّثَنَا هِشَامٌ ، عَنْ يَحْيَى ، عَنْ أَبِي سَلَمَةَ ، قَالَ سَأَلْتُ أَبَا سَعِيدٍ الْخُدْرِيَّ فَقَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَسْجُدُ فِي الْمَاءِ وَالطِّينِ حَتَّى رَأَيْتُ أَثَرَ الطِّينِ فِي جَبْهَتِهِ.
I saw Allah's Messenger (ﷺ) prostrating in mud and water and saw the mark of mud on his forehead.