Muwatta Imam Malik Book of Shortening the Prayer, Hadith 66

Yahya related to me from Malik that Abu Jafar al-Qari said, "I was praying, and Abdullah ibn Umar was behind me and I was not aware of it. Then I turned round and he prodded me (in disapproval)."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي جَعْفَرٍ الْقَارِئِ، أَنَّهُ قَالَ كُنْتُ أُصَلِّي وَعَبْدُ اللَّهِ بْنُ عُمَرَ وَرَائِي وَلاَ أَشْعُرُ فَالْتَفَتُّ فَغَمَزَنِي .