It is narrated from Sayyiduna Hudhayfah (may Allah be pleased with him), he says: I asked the Messenger of Allah (peace and blessings be upon him) about every single issue, until I even asked about what is the ruling on removing or cleaning pebbles from the place of prostration during prayer? So the Prophet (peace and blessings be upon him) said: You may remove them once, or it is better to just leave them as they are.