Sayyidah Aishah (may Allah be pleased with her) said: During the expedition of Tabuk, the Messenger of Allah (peace and blessings be upon him) was asked about the sutrah (screen) of a person praying, and he (peace and blessings be upon him) said: "Something equal to the back-rest of a camel saddle."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3453