بُلُوغُ الْمَرَامِ
Bulugh al-Maram 156
The Book of Prayer
وَلِمُسْلِمٍ مِنْ حَدِيثِ أَبِي مُوسَى: { فَأَقَامَ اَلْفَجْرَ حِينَ اِنْشَقَّ اَلْفَجْرُ, وَالنَّاسُ لَا يَكَادُ يَعْرِفُ بَعْضُهُمْ بَعْضًا }
Narrated Abu Musa (RA) in another version of Muslim: He (ﷺ) offered the Fajr prayer at daybreak when the people could hardly recognize one another.
Sunnah.com reference: Book 2, Hadith 6