Yahya related to me from Malik that he had heard that Umar ibnal-Khattab, Abdullah ibn Umar, Abdullah ibn Masud, Salim ibn Abdullah,al-Qasim ibn Muhammad, Ibn Shihab,and Sulayman ibn Yasar all said, "Ifa man has vowed to divorce his wife before marrying her and then hebreaks his vow, divorce is obligatory for him when he marries her."
USC-MSA web (English) reference: Book 29, Hadith 73
Arabic reference: Book 29, Hadith 1234