It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you prays, he should place something in front of himself. If he cannot find anything, then let him at least draw a line; then nothing that passes in front of him will harm him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3467