Hadithcore
Hadith #2105220

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.'

زَادَ أَحْمَدُ: { إِلَّا اَلْمَغْرِبَ فَإِنَّهَا وِتْرُ اَلنَّهَارِ, وَإِلَّا اَلصُّبْحَ, فَإِنَّهَا تَطُولُ فِيهَا اَلْقِرَاءَةُ } [1]‏ .‏

References2 variants
Sunnah.com reference
Book 2, Hadith 338
Arabic/English book reference
Book 2, Hadith 425