Hadith 503760
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ أَبِي جَعْفَرٍ الْقَارِئِ، أَنَّهُ قَالَ رَأَيْتُ عَبْدَ اللَّهِ بْنَ عُمَرَ إِذَا أَهْوَى لِيَسْجُدَ مَسَحَ الْحَصْبَاءَ لِمَوْضِعِ جَبْهَتِهِ مَسْحًا خَفِيفًا .
Yahya related to me from Malik that Abu Jafar al-Qari said, "I saw Abdullah ibn Umar quickly brush away the small stones from the place where he was going to put his forehead as he was going down into sajda."
Reference Muwatta Malik 503760
USC-MSA web (English) reference : Book 9, Hadith 45