Musnad Ahmad 1156:1

Hashim ibn Bashir narrated to us, Yahya ibn Sa'id narrated to us, from Sulayman ibn Yasar, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, that Subay'ah bint al-Harith gave birth twenty nights after the death of her husband, or thereabouts, and she wanted to remarry. Abu al-Sanabil said to her: "You may not do that until the later of the two terms has passed for you." She mentioned that to the Prophet, may Allah bless him and grant him peace, and he said: "She may marry if she wishes."