Musa bin Talhah bin Ubaydullah narrates from his father, Sayyiduna Talhah bin Ubaydullah (may Allah be pleased with him): We used to pray while animals kept passing in front of us. We mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "When there is something equal to the back-rest of a camel saddle in front of one of you (as a sutrah), then nothing that passes in front of him will harm him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3456