What is the ruling of Sharia on the phrase "May God enlighten you" (Allah yinawwir 'alayk)?

There is no harm in the phrase "May Allah illuminate your heart," as it is a supplication for a person to receive light. Although supplicating for someone who has done a kindness is permissible and encouraged, it is preferable to say "May Allah reward you with goodness" (Jazakallahu Khairan) in emulation of the Sunnah (definition). The Prophet, peace and blessings be upon him, supplicated for Tufail ibn Amr Ad-Dausi saying, "O Allah, illuminate him." Furthermore, the supplication "O Allah, illuminate for him in his grave" is mentioned in the funeral prayer.