Musnad Ahmad 1156:243

I read to Abd al-Rahman: Malik, from Abd Rabbih ibn Sa'id ibn Qays, from Abu Salamah ibn Abd al-Rahman, that he said: Abdullah ibn Abbas and Abu Hurayrah were asked about the woman whose husband has died. Ibn Abbas said: "The later of the two terms." And Abu Hurayrah said: "When she gives birth, she has become lawful [to remarry]." So Abu Salamah ibn Abd al-Rahman entered upon Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, and asked her about that. She said: Subay'ah al-Aslamiyyah gave birth half a month after the death of her husband. Two men proposed to her, one of them a youth and the other a middle-aged man. She inclined toward the youth. The middle-aged man said: "She has not become lawful." Her family was absent, and he hoped that when her family came, they would prefer him. She came to the Messenger of Allah, may Allah bless him and grant him peace, and he said: "You have become lawful, so marry whom you wish."