Maliks Muwatta Book 28, Hadith Number 19.

Yahya related to me from Malik that he had heard that when asked whether it was permissible for a man to return to his wife if he had divorced her irrevocably and then another man had married her after him and died before consummating the marriage, al-Qasim ibn Muhammad said, “It is not halal for the first husband to return to her.”

Malik said, about the muhallil, that he could not remain in the marriage until he undertook a new marriage. If he had intercourse with her in that marriage, she had her dowry.