Narrated Ibn 'Umar:
"While the Messenger of Allah (ﷺ) alive, we used to say: 'Abu Bakr, and (then) 'Umar, and (then) 'Uthman.' (ID)
English reference (ID): Vol. 1, Book 46, Hadith 3707 (ID)
Arabic reference (ID): Book 49, Hadith 4072 (ID)