Muwatta Malik, Book 9, Hadith 44 (USC-MSA)

Yahya related to me from Malik that he had heard that Abdullah ibn Umar would use the animal he was riding as a sutra when he prayed.

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَسْتَتِرُ بِرَاحِلَتِهِ إِذَا صَلَّى ‏.‏

sunnah.com

also referenced under: Book 9, Hadith 374 (In-Book)

Yahya related to me from Malik from Hisham ibn Urwa that his father used to pray in the desert without a sutra.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، أَنَّ أَبَاهُ، كَانَ يُصَلِّي فِي الصَّحْرَاءِ إِلَى غَيْرِ سُتْرَةٍ ‏.‏

sunnah.com

also referenced under: Book 9, Hadith 375 (In-Book)