مُوَطَّأُ مَالِكٍ
Muwatta Malik 396
Shortening the Prayer
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.
Arabic/English book reference: Book 9, Hadith 65