Hadith 503970
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي جَعْفَرٍ الْقَارِئِ، أَنَّهُ قَالَ كُنْتُ أُصَلِّي وَعَبْدُ اللَّهِ بْنُ عُمَرَ وَرَائِي وَلاَ أَشْعُرُ فَالْتَفَتُّ فَغَمَزَنِي .
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)."
Reference Muwatta Malik 503970
USC-MSA web (English) reference : Book 9, Hadith 66