Sayyiduna Utbah bin al-Nuddar (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "When your campaigns become distant, and the spoils of war are treated as lawful (for oneself), and compulsory orders (forced conscriptions) multiply, then your best jihad is ribat (guarding the frontiers)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14406