بُلُوغُ الْمَرَامِ
Bulugh al-Maram 355
The Book of Prayer
وَعَنْهَا قَالَتْ : { لَمْ يَكُنْ اَلنَّبِيُّ - صلى الله عليه وسلم -عَلَى شَيْءٍ مِنْ اَلنَّوَافِلِ أَشَدَّ تَعَاهُدًا مِنْهُ عَلَى رَكْعَتَيْ اَلْفَجْرِ } مُتَّفَقٌ عَلَيْهِ . [1]
Narrated ['Aishah (RA)]: The Prophet (ﷺ) was more particular about offering the two (optional) Rak'at at dawn than he was about offering any of the other optional prayers. [Agreed upon].
Sunnah.com reference: Book 2, Hadith 258