Daraqutni

Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: “There are four ways of giving divorce; two of them are lawful and two are unlawful. One lawful way is that a man gives divorce to a woman when she is in a state of purity (tuhr) and during that period of purity he has not had intercourse with her. (The second way is) a man gives divorce to a pregnant woman when her pregnancy has become apparent. (One) unlawful way is that a man gives divorce to a woman when she is in a state of menstruation. (The second unlawful way is) a man gives divorce to a woman in a state of purity after having had intercourse with her, and it is not known whether there is a child in the womb or not.”