Hadith 503960
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، لَمْ يَكُنْ يَلْتَفِتُ فِي صَلاَتِهِ .
Yahya related to me from Malik from Nafi that Ibn Umar would never turn around when praying.
Reference Muwatta Malik 503960
USC-MSA web (English) reference : Book 9, Hadith 65