Yahya related to me from Malik that Nafi said, "I stood behindAbdullah ibn Umar in one of the prayers when there was no one elsewith him, and he reached behind with his hand and placed me besidehim."
App reference: Book 8, Hadith 15
USC-MSA web (English) reference: Book 8, Hadith 15
Arabic reference: Book 8, Hadith 304