Volume 1 · Book 12 · Hadith 746
Narrated 'Abdullah bin Abi Qatada: My father said, "The Prophet used to prolong the first Rak'a of the Zuhr prayer and shorten the second one and used to do the same in the Fajr prayer."
— Sahih Bukhari, Volume 1, Book 12: Characteristics of Prayer, Hadith #746