Reference: Siyar A’laam an-Nubalaa – Volume 11, Page 177
Ibraaheem al-Harbee said that Ahmad [ibn Hanbal] was asked about [the permissibility of] a Muslim saying أَكْرَمَكَ الله (may Allaah nobilify you) to a Christian. He said:
Yes, [and] he should intend by it [his embracing] Islaam.
