بُلُوغُ الْمَرَامِ
Bulugh al-Maram 152
The Book of Prayer
وَلَهُ مِنْ حَدِيثِ بُرَيْدَةَ فِي اَلْعَصْرِ: { وَالشَّمْسُ بَيْضَاءُ نَقِيَّةٌ } [1] .
Narrated Buraidah (RA) in another version of the above Hadith regarding 'Asr (afternoon prayer): "When the sun is white and clear." [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 2