It wasnarrated that Abu Dharr said:
"The Messenger of Allah (ﷺ)said:'When anyone of you gets up to perform prayer, then indeed mercyisfacing him, so he should not smooth the pebbles.'" (ES)
English reference (ES): Vol. 1, Book 5, Hadith 1027 (ES)
Arabic reference (ES): Book 5, Hadith 1080 (ES)