Sahih Bukhari Volume 006, Book 060, Hadith Number 052.
Narrated By Al-Hasan : The sister of Ma'qal bin Yasar was divorced by her husband who left her till she had fulfilled her term of 'Iddat (i.e. the period which should elapse before she can Remarry) and then he wanted to remarry her but Maqal refused, so this Verse was revealed:
"Do not prevent them from marrying their (former) husbands." (2.232)