What is the ruling on saying: "O Possessor of Subtle Kindness, in You I seek refuge, and help, and You are sufficient for me"? And is it permissible to describe Allah as "Possessor of Subtle Kindness"?

The aforementioned supplication is correct in meaning, but it is not authentically transmitted from the Prophet, peace and blessings be upon him, and his companions. There is no prohibition in the phrase "al-lutf al-khafi" (the hidden subtlety), and scholars have used it. Its attribution to Ali ibn Abi Talib, may Allah be pleased with him, has become widespread. The scope of informing about Allah is broader than the scope of affirming names and attributes, so it is not a condition that the wordings be prescriptive (tawqifiyyah). The meaning of "al-lutf al-khafi" is correct, for Allah Almighty shows subtle kindness to His servants in ways hidden from them, and then its effects become manifest later, as Al-Sa'di explained in his exegesis of Allah's name "Al-Latif" (The Subtle).