Is there a supplication to avoid nocturnal emission, especially when intending to fast?

There is no authentic supplication to prevent nocturnal emission. There is no need to fear it when intending to fast, for whoever wakes up in a state of major ritual impurity while fasting, their fast is valid. Similarly, whoever experiences nocturnal emission during the day in Ramadan (definition) should complete their fast, and there is no blame upon them. It was narrated from Aisha that the Prophet, peace and blessings be upon him, would wake up in a state of major ritual impurity while intending to fast.