From Sahih Bukhari (Hadith 5318):
Narrated Um Salama:(the wife of the Prophet) A lady from Bani Aslam, called Subai'a, become a widow while she was pregnant. Abu As-Sanabil bin Ba'kak demanded her hand in marriage, but she refused to marry him and said, "By Allah, I cannot marry him unless I have completed one of the two prescribed periods." About ten days later (after having delivered her child), she went to the Prophet (ﷺ) and he said (to her), "You can marry now
Reference: Bukhari 5318