بُلُوغُ الْمَرَامِ
Bulugh al-Maram 430
The Book of Prayer
زَادَ أَحْمَدُ: { إِلَّا اَلْمَغْرِبَ فَإِنَّهَا وِتْرُ اَلنَّهَارِ, وَإِلَّا اَلصُّبْحَ, فَإِنَّهَا تَطُولُ فِيهَا اَلْقِرَاءَةُ } [1] .
Ahmad added, 'Except the Maghrib (prayer) for it is the Witr (prayer) of the day; and except the Fajr (prayer), since the recitation (of the Qur'an) is prolonged in it.'
Sunnah.com reference: Book 2, Hadith 338