Hadithcore
Mishkat al-Masabih, 634

Ibn Mas'ud and Samura b. Jundub reported God's Messenger as saying, "The middle prayer is the afternoon prayer.” Tirmidhi transmitted it.

عَن ابْن مَسْعُود وَسمرَة بن جُنْدُب قَالَا: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «صَلَاةُ الْوُسْطَى صَلَاةُ الْعَصْرِ» . رَوَاهُ التِّرْمِذِيُّ

References1 variant
In-Book Reference
Book 4, Hadith 67