Musnad Abu Ya'la al-Mawsili 2:98

Ishaq ibn Isma'il al-Taliqani narrated to us, Jarir narrated to us, from Husayn, from Salim ibn Abi al-Ja'd, who said: Umar said—and he mentioned some words—"Indeed, people say, 'If only you would appoint a successor.' But I do not find anyone more worthy of this matter than these individuals whom the Prophet, peace and blessings of Allah be upon him, passed away while he was pleased with them." "So, whomever they appoint as successor, he is the Caliph after me."