كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 305
Ahmad and ad-Daraqutni narrated something similar, adding that he (ﷺ) continued to recite Qunut (supplication in prayer) in the Fajr prayer till he separated from this world.
وَلِأَحْمَدَ وَاَلدَّارَقُطْنِيِّ نَحْوُهُ مِنْ وَجْهٍ آخَرَ , وَزَادَ : { فَأَمَّا فِي اَلصُّبْحِ فَلَمْ يَزَلْ يَقْنُتُ حَتَّى فَارَقَ اَلدُّنْيَا } .‏ 1
Reference : Bulugh al-Maram 305
In-book reference : Book 2, Hadith 305
English translation : Book 2, Hadith 303
Hadith Text
Reference