بُلُوغُ الْمَرَامِ
Bulugh al-Maram 171
The Book of Prayer
وَعَنْ اِبْنِ مَسْعُودٍ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ أَفْضَلُ اَلْأَعْمَالِ اَلصَّلَاةُ فِي أَوَّلِ وَقْتِهَا } رَوَاهُ اَلتِّرْمِذِيُّ, وَالْحَاكِمُ. وَصَحَّحَاهُ [1] .
Narrated Ibn Mas'ud (RA): Allah's Messenger (ﷺ) said: "One of the best deeds is to offer Salat (prayer) in its early time." [Reported by at-Tirmidhi and al-Hakim who (both) graded it Sahih].
Sunnah.com reference: Book 2, Hadith 23