كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 355
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].
وَعَنْهَا قَالَتْ : { لَمْ يَكُنْ اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-عَلَى شَيْءٍ مِنْ اَلنَّوَافِلِ أَشَدَّ تَعَاهُدًا مِنْهُ عَلَى رَكْعَتَيْ اَلْفَجْرِ } مُتَّفَقٌ عَلَيْهِ .‏ 1
Reference : Bulugh al-Maram 355
In-book reference : Book 2, Hadith 355
English translation : Book 2, Hadith 350
Hadith Text
Reference