Jami At Tirmidhi
544
Sahih+3

Ibn Umar narrated:"I traveled with the Prophet, Abu Bakr, Umar, and Uthman; they would pray Zuhr and Asr as two Rak'ah and two Rak'ah, not praying before them nor after them." And Ibn Umar said: "If I was going to pray before it or after it then I would pray it complete

SahihAhmad Muhammad Shakir
SahihAl-Albani