´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).
Hadith Referenceسنن نسائي / كتاب الطلاق / 3552
Hadith Gradingالألبانی:صحيح الإسناد | زبیر علی زئی:ضعيف، إسناده ضعيف، إبراهيم النخعي عنعن. وانظر سنن أبى داود (2307 بتحقيقي). والحديث السابق (الأصل: 3551) يغني عنه. انوار الصحيفه، صفحه نمبر 347
(1) In this hadith of Imam al-Nasa'i rahimahullah, there are two teachers: Muhammad ibn Miskin and Maymun ibn Abbas. These words are from Maymun.
(2) "Mubahalah" means: upon the liar be the curse. It is as if they had complete certainty that the waiting period ('iddah) of a pregnant woman is until childbirth (wada' al-haml).
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 3552