Abu al-Walid said: I asked Ibn 'Umar about the gravel spread pin the mosque. He replied: One night the rain fell and the earth was moistened. A man was bringing the gravel (broken stones) in his cloth and spreading it beneath him. When the Messenger of Allah (ﷺ) finished his prayer, he said: How fine it is !
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا سَهْلُ بْنُ تَمَّامِ بْنِ بَزِيعٍ، حَدَّثَنَا عُمَرُ بْنُ سُلَيْمٍ الْبَاهِلِيُّ، عَنْ أَبِي الْوَلِيدِ، سَأَلْتُ ابْنَ عُمَرَ عَنِ الْحَصَى الَّذِي، فِي الْمَسْجِدِ فَقَالَ مُطِرْنَا ذَاتَ لَيْلَةٍ فَأَصْبَحَتِ الأَرْضُ مُبْتَلَّةً فَجَعَلَ الرَّجُلُ يَأْتِي بِالْحَصَى فِي ثَوْبِهِ فَيَبْسُطُهُ تَحْتَهُ فَلَمَّا قَضَى رَسُولُ اللَّهِ صلى الله عليه وسلم الصَّلاَةَ قَالَ " مَا أَحْسَنَ هَذَا " .
sunnah.com