Bulugh al-Maram 1 The Book of Purification كتاب الطهارة 145

وَلَهُمَا فِي حَدِيثِ مَيْمُونَةَ: { ثُمَّ أَفْرَغَ عَلَى فَرْجِهِ, فَغَسَلَهُ بِشِمَالِهِ, ثُمَّ ضَرَبَ بِهَا اَلْأَرْضَ }

Narrated Maimuma (rad): Allah’s Messenger (صلى الله عليه وسلم) poured water over his private parts and washed them with his left hand. He then struck his hand against the earth.