بُلُوغُ الْمَرَامِ
Bulugh al-Maram 161
The Book of Prayer
وَعَنْ أَبِي هُرَيْرَةَ - رضى الله عنه - أَنَّ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -قَالَ: { مَنْ أَدْرَكَ مِنْ اَلصُّبْحِ رَكْعَةً قَبْلِ أَنْ تَطْلُعَ اَلشَّمْسُ فَقَدْ أَدْرَكَ اَلصُّبْحَ, وَمَنْ أَدْرَكَ رَكْعَةً مِنْ اَلْعَصْرِ قَبْلَ أَنْ تَغْرُبَ اَلشَّمْسُ فَقَدْ أَدْرَكَ اَلْعَصْرَ } مُتَّفَقٌ عَلَيْهِ [1] .
Narrated Abu Hurairah (RA): The Prophet (ﷺ) said: "He who prays a Rak'a of the Fajr prayer before the sun rises, has offered the dawn prayer in its time, and he who prays a Rak'a of the 'Asr prayer before the sun sets has offered the afternoon prayer." [Agreed upon].
Sunnah.com reference: Book 2, Hadith 11