Muwatta Malik, Book 9, Hadith 65 (USC-MSA) ⓘ
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 .
also referenced under: Book 9, Hadith 396 (In-Book)