Muwatta Imam Malik Book of Shortening the Prayer, Hadith 44

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 .