The Book of the Times (of Prayer)
6/27. Chapter:
Al-Isfar (Praying fajr When It Has Become Lighter)
Sunan an-Nasai
أَخْبَرَنِي إِبْرَاهِيمُ بْنُ يَعْقُوبَ، قَالَ حَدَّثَنَا ابْنُ أَبِي مَرْيَمَ، قَالَ أَخْبَرَنَا أَبُو غَسَّانَ، قَالَ حَدَّثَنِي زَيْدُ بْنُ أَسْلَمَ، عَنْ عَاصِمِ بْنِ عُمَرَ بْنِ قَتَادَةَ، عَنْ مَحْمُودِ بْنِ لَبِيدٍ، عَنْ رِجَالٍ، مِنْ قَوْمِهِ مِنَ الأَنْصَارِ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " مَا أَسْفَرْتُمْ بِالْفَجْرِ فَإِنَّهُ أَعْظَمُ بِالأَجْرِ " .
It was narrated from Mahmud bin Labid, from some men among his people who were of the Ansar, that the Messenger of Allah (ﷺ) said: "The more you delay Fajr, the greater the reward."
Reference : Sunan an-Nasa'i 549In-book reference : Book 6, Hadith 56English translation : Vol. 1, Book 6, Hadith 550