Ubayd bin Umayr al-Laythi (may Allah have mercy on him) said: I asked Ibn Umar (may Allah be pleased with them both): "Why is it that I see you crowding at these two corners, and I have not seen anyone else among the Companions (may Allah be pleased with them) crowding at them besides you?" He said: "I heard the Messenger of Allah (peace and blessings be upon him) say: 'Touching them wipes away sins.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9260