Imam al-Sha'bi narrated: Sayyiduna Umar (may Allah be pleased with him) wrote in his will: "Do not let any official appointed by me remain in his post for more than one year, but keep al-Ash'ari in his post for four years."
Ahmad recorded it with a hasan (good) chain, except that Imam al-Sha'bi did not hear from Sayyiduna Umar (may Allah be pleased with him).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 15946