Yahya related to me from Malik that Abu Jafar al-Qari said, "Iwas praying, and Abdullah ibn Umar was behind me and I was not awareof it. Then I turned round and he prodded me (in disapproval)." (IT)
USC-MSA web (English) reference: Book 9, Hadith 66 (IT)
Arabic reference (IT): Book 9, Hadith 397 (IT)