بُلُوغُ الْمَرَامِ
Bulugh al-Maram 305
The Book of Prayer
وَلِأَحْمَدَ وَاَلدَّارَقُطْنِيِّ نَحْوُهُ مِنْ وَجْهٍ آخَرَ , وَزَادَ : { فَأَمَّا فِي اَلصُّبْحِ فَلَمْ يَزَلْ يَقْنُتُ حَتَّى فَارَقَ اَلدُّنْيَا } . [1]
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.
Sunnah.com reference: Book 2, Hadith 191