سنن النسائي

It was narrated from 'Alqamah bin Qais that Ibn Mas'ud said: "Whoever wants, I will meet and debate with him and invoke the curse of Allah upon those who lie. The Verse: 'And for those who are pregnant (whether they are divorced or their husbands are dead), their 'Iddah (prescribed period) is until they lay down their burden.' was only revealed after the Verse about women whose husbands die. 'When a woman whose husband has died gives birth, it becomes permissible for her to marry.'" This is the wording of Maimun (one of the narrators).