مُوَطَّأُ مَالِكٍ
Muwatta Malik 375
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، أَنَّ أَبَاهُ، كَانَ يُصَلِّي فِي الصَّحْرَاءِ إِلَى غَيْرِ سُتْرَةٍ .
Yahya related to me from Malik from Hisham ibn Urwa that his father used to pray in the desert without a sutra.
Arabic/English book reference: Book 9, Hadith 44