The oath [ilaa] is in the Law; it is a solemn promise to abstain from intercourse with women. Its basis is in the word of Allah Most High { For those who take an oath to be apart from their women, four months on standby } [Al-baqara 226]. Ubay ibn Ka’b and Ibn ‘Abbas recited it as “those who swear”.
… since four months is the length of time at which a woman suffers harm from deferring intercourse from her.
… the one who has taken oath, if he is summoned and requested to return, and he is able to, but does not, then he is ordered to divorce.