Does inserting the tip of the middle finger slightly into the anus while cleaning oneself (Istinja') for the purpose of purification invalidate the fast of a fasting person?

The obligation in istinja' (cleansing oneself after defecation or urination) is to purify the area until its roughness returns. It is not prescribed to insert a finger into the inner part of the private organ. Sheikh Al-Islam Ibn Taymiyyah and those who agreed with him hold the view that inserting a finger does not invalidate the fast, and this is a strong opinion. However, it is more cautious to make up the fast for one who deliberately did so while knowing the ruling. As for someone who did it out of forgetfulness or ignorance, their fast is not invalidated, and they are not required to make it up.