Musannaf Ibn Abi Shaybah 27532 — A man who writes: Amma ba'd

Abdullah bin Hubayrah said that Sayyiduna Abu al-Darda (may Allah be pleased with him) wrote in a letter to Sayyiduna Salman (may Allah be pleased with him): "Amma ba'd (to proceed)," and Sayyiduna Salman wrote in a letter to Sayyiduna Abu al-Darda: "Amma ba'd."