Anas ibn Malik reported that people knocked on the door of the Prophet, may Allah bless him and grant him peace, using their fingernails.  

عَنْ أَنَسِ بْنِ مَالِكٍ إِنَّ أَبْوَابَ النَّبِيِّ ﷺ كَانَتْ تُقْرَعُ بِالأظَافِيرِ