[Machine] From the Messenger of Allah ﷺ and then from the successors after him, it is narrated that they were devout in the Fajr prayer. It is well known from Umar in authentic narrations that he used to be devout in the Fajr prayer. So if they sometimes neglected it unintentionally or intentionally, that indicates that it is not obligatory. And when it is not reported from any of them that they performed the prostrations of forgetfulness for it, that indicates that there is no prostration of forgetfulness for it. And Allah knows best.  

عَنْ رَسُولِ اللهِ ﷺ ثُمَّ عَنِ الْخُلَفَاءِ بَعْدَهُ أَنَّهُمْ قَنَتُوا فِي صَلَاةِ الصُّبْحِ وَمَشْهُورٌ عَنْ عُمَرَ مِنْ أَوْجُهٍ صَحِيحَةٍ أَنَّهُ كَانَ يَقْنُتُ فِي صَلَاةِ الصُّبْحِ فَلَئِنْ تَرَكُوهُ فِي بَعْضِ الْأَحَايِينِ سَهْوًا أَوْ عَمْدًا دَلَّ ذَلِكَ عَلَى كَوْنِهِ غَيْرَ وَاجِبٍ وَحِينَ لَمْ يَنْقُلْ عَنْ أَحَدٍ مِنْهُمْ أَنَّهُ سَجَدَ سَجْدَتَيِ السَّهْوِ لِذَلِكَ دَلَّ عَلَى أَنَّهُ لَا سُجُودَ فِي السَّهْوِ عَنْهُ وَاللهُ أَعْلَمُ