Hakeemul Ummat Hadhrat Maulana Muhammad Ashraf Ali Thanvi (qudduhus sirruhu) mentions,
At the time of departure some people instead of “Asalam ‘Alaykum” say only ‘Khuda Hafiz’. Although this is changing of Shari’at, which is a big sin. Instead making the habit of saying ‘Khuda Hafiz’ after Salam is even forbidden. (Ahsaanul Fatawa, vol. 1 pg. 385)
[Taken from "Aghlatul-Awaam (Awaam ki Ghalat Masa'il)"]
Filed under: 2. Fiqh, 4. Social Matters

JazakAllah for such great information.