What to say to the disbeliever if he sneezes and praises Allah
“May Allah guide you and set your affairs in order.”
يَهْـديكُـمُ اللهُ وَيُصْـلِحُ بالَـكُم
Yahdeekumullaahu wa yuslihu baalakum.
At-Tirmidhi 5/82, Ahmad 4/400, Abu Dawud 4/ 308. See also Al-Albani, Sahih At-Tirmidhi 2/354.