Ila' is a type of vow where a man swears not to sleep with his wife for a certain period, whether less or more than four months. If the vow of Ila' was for less than four months, the man has to wait for the vow's period to end and then is allowed to have sexual intercourse with his wife. She has to be patient and she cannot ask her husband, in this case, to end his vow before the end of its term. It is reported in the Two Sahihs that `A'ishah said that Allah's Messenger ﷺ swore he would stay away from with his wives for a month. He then came down after twenty-nine days saying:
«الشَّهْرُ تِسْعٌ وَعِشْرُون»
(The (lunar) month is twenty-nine days.)
Similar was narrated by `Umar bin Al-Khattab and reported in the Two Sahihs. If the period of Ila' is for more than four months, the wife is allowed in this case to ask her husband, upon the end of the four months, to end the Ila' and have sexual relations with her. Otherwise, he should divorce her, by being forced to do so by the authorities if necessary, so that the wife is not harmed. Allah said:
لِّلَّذِينَ يُؤْلُونَ مِن نِّسَآئِهِمْ
(Those who take an oath not to have sexual relations with their wives) meaning, swear not to have sexual relations with the wife. This Ayah indicates that the Ila' involves the wife and not a slave-women, as the majority of the scholars have agreed,
تَرَبُّصُ أَرْبَعَةِ أَشْهُرٍ
(...must wait for four months,) meaning, the husband waits for four months from the time of the vow and then ends the Ila' (if the vow was for four or more months) and is required to either return to his wife or divorce her. This is why Allah said next:
فَإِن فَآءُوا
(...then if they return,) meaning, to a normal relationship, having sexual intercourse with the wife. This is the Tafsir of Ibn `Abbas, Masruq, Ash-Sha`bi, Sa`id bin Jubayr and Ibn Jarir.
فَإِنَّ اللَّهَ غَفُورٌ رَّحِيمٌ
(...verily, Allah is Oft-Forgiving, Most Merciful.) with any shortcomings that occurred in the rights of the wife because of the vow of Ila'.
Allah said:
وَإِنْ عَزَمُواْ الطَّلَـقَ
(And if they decide upon divorce,) indicating that divorce does not occur by merely passing the four month mark (during the Ila'). Malik reported from Nafi` that `Abdullah bin `Umar said, "If the man swears to Ila' from his wife, then divorce does not occur automatically even after the four months have passed. When he stops at the four months mark, he should either divorce or return." Al-Bukhari also reported this Hadith. Ibn Jarir reported that Suhayl bin Abu Salih said that his father said, "I asked twelve Companions about the man who does Ila' with his wife. They all stated that he does not have to do anything until the four months have passed and then has to either retain or divorce her." Ad-Daraqutni also reported this from Suhayl.
It is also reported from `Umar, `Uthman, `Ali, Abu Ad-Darda', `A'ishah, Ibn `Umar and Ibn `Abbas. This is also the opinion of Sa`id bin Musayyib, `Umar bin `Abdul-`Aziz, Mujahid, Tawus, Muhammad bin Ka`b and Al-Qasim.