Ibrahim, the son of the Prophet ﷺ, died when he was eighteen months old. The Messenger of Allah ﷺ did not pray over him.  

مَاتَ إِبْرَاهِيمُ ابْنُ النَّبِيِّ ﷺ وَهُوَ ابْنُ ثَمَانِيَةَ عَشَرَ شَهْرًا فَلَمْ يُصَلِّ عَلَيْهِ رَسُولُ اللَّهِ ﷺ  

When Ibrahim, the son of the Prophet ﷺ died, he prayed over him at the place where he used to sit. Abu Dawud said: I recited to Sa'id b. Ya'qub al-Taliqani saying: Ibn al-Mubarak transmitted to you from Ya'qub b. al-Qa'qa' on the authority of 'Ata that the Prophet ﷺ prayed over his son Ibrahim when he was seventeen days old.  

لَمَّا مَاتَ إِبْرَاهِيمُ ابْنُ النَّبِيِّ ﷺ صَلَّى عَلَيْهِ رَسُولُ اللَّهِ ﷺ فِي الْمَقَاعِدِ