Jami` at-Tirmidhi 2225
جامع الترمذي#2225SahihSalim bin 'Abdullah narrated from his father who said:"It was said to 'Umar bin Al-Khattab: 'Perhaps you should endorse your successor.' He said: 'If I appoint a successor, then indeed Abu Bakr appointed a successor. And if I do not appoint a successor, the Messenger of Allah(s.a.w) did not appoint a successor
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 33:68