Mujahid (may Allah have mercy on him) says concerning the saying of Allah Almighty: "And do not retain them to harm them" [Surah al-Baqarah: 231], that harm (dirar) is that a husband divorces his wife and, one day before the end of the iddah, takes her back and then divorces her, and keeps doing so in the same way; this is causing harm.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15155