Haram bin Mu'awiyah (may Allah have mercy on him) says that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) wrote: "Train the horses, but let the cross not be raised among you, nor let pigs live near you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18713