Musnad Ahmad 659:0

Narrated Abu al-Sanabil ibn Ba'kak: The Prophet (ﷺ) said: "The waiting period (iddah) of the woman whose husband has died is four months and ten days." Subay'ah bint al-Harith came to him while she was pregnant, and she asked him for a verdict. He gave her a verdict that she should marry when she had delivered her child.