Muwatta Imam Malik Book of Shortening the Prayer, Hadith 65

Yahya related to me from Malik from Nafi that Ibn Umar would never turn around when praying.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، لَمْ يَكُنْ يَلْتَفِتُ فِي صَلاَتِهِ .