Is it permissible to say, 'Fee amaanillah' to someone when leaving him?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

It is permissible for a Muslim to say to the one whom he bids farewell or to any person, “Fee amaanillaah (in the protection of Allaah)”; because the meaning is supplicating for him to be in the security, protection and care of Allaah, and it is better that he says this after saying salaam to him...

Shaykh Ibn ‘Uthaymeen said:

“If one wants to depart he says, 'Fee amaanillaah' or Ma’as-salaamah' or 'Oudi’ouk-Allaah', and so forth. There is nothing wrong with these expressions, but they should be coupled with saying salaam, because it is legislated for a person when he leaves to say salaam in the same manner as when he meets a person. He says for example when he wants to leave, 'Assalamu alaykum, fee amaanillaah,' and so forth. When the Prophet said good bye to a traveler, he used to say, 'I place your religion, your faithfulness, and the ends of your deeds in the trust of Allaah.'”

For more benefit, please refer to fatwa 166001.

Allaah knows best.