What is the ruling on some du’aa’s that have not been narrated (from the Prophet (peace and blessings of Allaah be upon him)), such as, “Rocky obstacle and falling star, may the eye of the envier come back on him and on those who are dearest to him.”

Praise be to Allaah. 

This du’aa’ has no basis. It
involves enmity against people other than the aggressor, so it is not
permissible to use it because the Prophet
(peace and blessings of Allaah be upon him) said: “Whoever does an action
that is not in agreement with this matter of ours (i.e., Islam), will
have it rejected.” And Allaah knows best.