Abu Ahwas narrated this in the gathering of Sa'id bin al-Musayyib. At that time, Sa'id bin al-Musayyib was also sitting there. Abu Ahwas reported: He heard Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him) narrate this. The Prophet Muhammad (peace and blessings be upon him) said: "When a person stands to perform prayer, mercy faces him. Therefore, (while performing prayer) he should not move the pebbles (the narrator is in doubt, perhaps these are the words:) nor touch the pebbles."
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2274
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيضعيف - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطهو مكرر ما قبله.