"I accompanied the Messenger of Allah on eighteen journeys, and I did not see him leave the two Rak'ah when the sun waned before Zuhr."
Jami` at-Tirmidhi Book 6, Hadith 7
← Previous Hadith "The Messenger of Allah traveled on a journey and he prayed two Rak'ah for nineteen days." Ibn Abbas said: "So when we would stay somewhere for nineteen (days) we would pray two Rak'ah , and if we stayed longer than that we would complete the Salat."

Jami` at-Tirmidhi Book 6, Hadith 6 Next Hadith → "I prayed Zuhr with the Prophet on a journey as two Rak'ah, and two Rak'ah after it."

Jami` at-Tirmidhi Book 6, Hadith 8