(a) Ya'la said: I was performing tawaf with Umar (may Allah be pleased with him). When we reached the two western corners, I said: "Why do you not touch them?" and I placed myself between him and the wall. He said: "Did you not perform tawaf with the Messenger of Allah (peace and blessings be upon him)?" I said: "Of course." He said: "Did you see the Messenger of Allah (peace and blessings be upon him) touching them?" I said: "No." He said: "Then for you there is 'in the Messenger of Allah an excellent example' [Al-Ahzab: 21], so follow it."
(b) Imam al-Shafi'i (may Allah have mercy on him) said: The reason concerning those two, as we see it, is that the House was not completed on the foundations of Ibrahim (peace be upon him), so they became like the rest of the House.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9242