Abu Salamah bin 'Abdur-Rahman said: "Ibn 'Abbas, Abu Hurairah and I were together, and Ibn 'Abbas said: 'If a woman gives birth after her husband dies, her 'Iddah is the longer of the two periods.'" Abu Salamah said: "We sent Kuraib to Umm Salamah to ask her about that. He came to us and told us from her that the husband of Subai'ah died and she gave birth a few days after her husband died, and the Messenger of Allah (ﷺ) told her to get married."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا حُسَيْنُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا جَعْفَرُ بْنُ عَوْنٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، قَالَ أَخْبَرَنِي سُلَيْمَانُ بْنُ يَسَارٍ، قَالَ أَخْبَرَنِي أَبُو سَلَمَةَ بْنُ عَبْدِ الرَّحْمَنِ، قَالَ كُنْتُ أَنَا وَابْنُ، عَبَّاسٍ وَأَبُو هُرَيْرَةَ فَقَالَ ابْنُ عَبَّاسٍ إِذَا وَضَعَتِ الْمَرْأَةُ بَعْدَ وَفَاةِ زَوْجِهَا فَإِنَّ عِدَّتَهَا آخِرُ الأَجَلَيْنِ . فَقَالَ أَبُو سَلَمَةَ فَبَعَثْنَا كُرَيْبًا إِلَى أُمِّ سَلَمَةَ يَسْأَلُهَا عَنْ ذَلِكَ فَجَاءَنَا مِنْ عِنْدِهَا أَنَّ سُبَيْعَةَ تُوُفِّيَ عَنْهَا زَوْجُهَا فَوَضَعَتْ بَعْدَ وَفَاةِ زَوْجِهَا بِأَيَّامٍ فَأَمَرَهَا رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ تَتَزَوَّجَ .
sunnah.com