Is, "O Allah, I do not ask You to avert the decree, but I ask You for gentleness within it," a Hadith from the Messenger of Allah, may Allah’s prayers and peace be upon him?

The aforementioned Hadith (definition) is neither authentic nor does it have any basis. It contradicts what has been established from the Prophet, peace and blessings be upon him, that supplication repels divine decree. It has been narrated from Ibn Abbas that supplication pushes back destiny, and that Allah erases by supplication whatever He wills from destiny. It has also been authentically reported that Umar ibn al-Khattab made a supplication asking for the erasure of wretchedness and the affirmation of happiness. Furthermore, supplication must be resolute, not contingent upon Allah's will.