What constitutes penetration sufficient to lift the impediment of 'Innah' (impotence)?
Chapter on the Term for the Impotent and the Castrated who is not amputated
Al-Mughni
Book of Marriage
Primary text
The impediment of 'Innah' is removed by the insertion of the glans penis (hashafah) into the vagina. All rulings related to sexual intercourse are contingent upon the insertion of the glans, thus establishing a valid coitus. This view is established because 'Innah' signifies the inability to perform intercourse, and this inability ceases upon the actual occurrence of intercourse.
Supporting text
If the penis is circumcised (lacking the glans), there are two views. One is that 'Innah' is not lifted unless the entirety of the remaining part is inserted, as there is no standard measure to adopt other than the whole remaining part. The second view holds that the measure equivalent to the size of the glans should be inserted, ensuring the sufficiency of the circumcised male is comparable to the sound male. Al-Shafi'i holds two opinions corresponding to these two views.