Musnad Ahmad 1279:1

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah, who said: Marwan sent 'Abd Allah ibn 'Utbah to Subay'ah bint al-Harith to ask her about what the Messenger of Allah (peace and blessings of Allah be upon him) had issued as a legal verdict for her. She informed him that she was married to Sa'd ibn Khawlah, and he died while she was with him during the Farewell Pilgrimage, and he was a veteran of Badr. She delivered her pregnancy before four months and ten days had passed since his death. When she had recovered from her postpartum bleeding and had applied kohl, Abu al-Sanabil—meaning Ibn Ba'kak—met her and said to her: "Control yourself"—or something similar—"perhaps you desire marriage? It is four months and ten days from the death of your husband." She said: So I came to the Prophet (peace and blessings of Allah be upon him) and mentioned to him what Abu al-Sanabil ibn Ba'kak had said. The Prophet (peace and blessings of Allah be upon him) said to her: "You have become lawful [to marry] when you delivered your pregnancy."