Maliks Muwatta Book 29, Hadith Number 84.

Yahya related to me from Malik from Nafi that Abdullah ibn Umar was asked about a woman whose husband died while she was pregnant, and he said, “When she gives birth, she is free to marry.” A man of the Ansar who was with him told him that Umar ibn al-Khattab had said, “Had she given birth while her husband was still on his bed, unburied, she would be free to marry.”