The Messenger of Allah ﷺ was more particular about observing the supererogatory rak'ahs before the dawn prayer than about observing any of the other supererogatory prayers.  

إِنَّ رَسُولَ اللَّهِ ﷺ لَمْ يَكُنْ عَلَى شَىْءٍ مِنَ النَّوَافِلِ أَشَدَّ مُعَاهَدَةً مِنْهُ عَلَى الرَّكْعَتَيْنِ قَبْلَ الصُّبْحِ