From whom is Iila' (oath of abstinence) valid concerning the wife?

Primary text

The oath of abstinence (Iila') is valid regarding every wife, whether Muslim or Dhimmi, free or slave. This is due to the generality of the Divine statement: "For those who swear abstinence from their wives is a waiting period of four months" (Quran 2:226). Furthermore, since every one of them is a wife, the Iila' is valid from her just as it is from a free Muslim woman. The Iila' is valid both before consummation and after consummation. This view is held by Al-Nakha'i, Malik, Al-Awza'i, and Al-Shafi'i. The evidence supporting the generality of the verse and the underlying rationale is that the husband swears to abstain from sexual intercourse with his wife, thus resembling the situation after consummation.