Hadith 164

Bulugh al-Maram

Book 8: Marriage
وَفِي لَفْظٍ: { أَنَّهَا وَضَعَتْ بَعْدَ وَفَاةِ زَوْجِهَا بِأَرْبَعِينَ لَيْلَةً 1‏ } .‏
A wording has:

"She gave birth to a child after forty nights of her husband's death."