The Messenger of Allah ﷺ said, "When one of you stands for prayer, mercy faces him directly, so let him not wipe away the pebbles."
حدثنا هشام بن عمار ومحمد بن الصباح قالا حدثنا سفيان بن عيينة عن الزهري عن أبي الأحوص الليثي
قال رسول الله ﷺ إذا قام أحدكم إلى الصلاة فإن الرحمة تواجهه فلا يمسح بالحصى
[Its chain of transmission is hasan] Abu Dawud narrated it in the Book of Prayer, chapter on wiping away pebbles during prayer, no. 945, from the hadith of Sufyan, through him. Tirmidhi graded it hasan, and Ibn Khuzayma and Ibn Hibban graded it sahih, as did the Hafiz in Bulugh Maram, no. 238, 239, chapter on the encouragement to have khushu in prayer.