كتاب قصر الصلاة فى السفر
9  
Shortening the Prayer

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 ‏.‏
USC-MSA web (English) reference : Book 9, Hadith 44
Arabic reference : Book 9, Hadith 374