Is it permissible to complete a fast after having eaten forgetfully, in the case of a voluntary fast intended for days, or intended at night?

Firstly: Whoever intended to fast from the night and then ate out of forgetfulness should complete their fast, according to the Prophet's (peace be upon him) saying: "Whoever forgets while fasting, and eats or drinks, let him complete his fast, for it is Allah who has fed him and given him drink."

Secondly: It is permissible to fast a voluntary fast with an intention made during the day, for one who has not eaten, drunk, or done anything that breaks the fast, based on the hadith of Aisha (may Allah be pleased with her): "Then I am fasting." The condition for the validity of this is that one must not have eaten anything or done anything that breaks the fast. As for someone who ate after dawn – even if forgetfully – it is not permissible for him to intend to fast at that point.

Thirdly: An intention made a day or more before the night of fasting is not valid. Therefore, whoever did not make the intention during the night and then woke up and ate, is not considered fasting and does not fall under the ruling of one who ate out of forgetfulness.