وَعَنْهَا قَالَتْ: «لَمْ يَكُنْ اَلنَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَلَى شَيْءٍ مِنْ اَلنَّوَافِلِ أَشَدَّ تَعَاهُدًا مِنْهُ عَلَى رَكْعَتَيْ اَلْفَجْرِ» مُتَّفَقٌ عَلَيْهِ. 11 صحيح
The Prophet (sallallahu alayhi wa sallam) was more particular about offering the two (optional) Rakat at dawn than he was about offering any of the other optional prayers.
Bulugh al-Maram 433, Book 2, The Book of Prayer, Hadith 254. 1 min read