Al-Matalib al-'Aliyah 2654 — Greeting disbelievers with salam

It is narrated from Imam Ibrahim al-Nakha'i (may Allah have mercy on him) that he said: "When you have some business or need with a person, begin with the salam (before starting what you have to say)." And Imam Mujahid (may Allah have mercy on him) said: "When you write (a letter or message), write in it: 'As-salamu 'ala man ittaba'a al-huda' (Peace be upon whoever follows the guidance)."
Imam Ibn Abi Shaybah recorded this athar (report) in his al-Musannaf; it explains the etiquette of speaking for one who has a request and the Shari'ah manner of letter writing, namely that the salam before speech is a good Sunnah.