Nafi' narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he did not mind whether the ish'ar was done on the right side or the left.
Imam al-Shafi'i (may Allah have mercy on him) said, in other than this narration: The ish'ar should be done on the right side of the hump, because that is how the Messenger of Allah (peace and blessings be upon him) performed the ish'ar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10173