Muwatta Malik
Book 9, Hadith 65
Arabic hadith text
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، لَمْ يَكُنْ يَلْتَفِتُ فِي صَلاَتِهِ .
Translation
Yahya related to me from Malik from Nafi that Ibn Umar wouldnever turn around when praying.