عَنْ مُجَالِدٍ عَنِ الشَّعْبِيِّ قَالَ كَتَبَ عُمَرُ فِي وَصِيَّتِهِ أَنْ لَا يُقَرَّ لِي عَامِلٌ أَكْثَرَ مِنْ سَنَةٍ وَأَقِرُّوا الْأَشْعَرِيَّ يَعْنِي أَبَا مُوسَى أَرْبَعَ سِنِينَ
It is narrated from Sha'bi, he says: In his will, Sayyiduna Umar bin Khattab (may Allah be pleased with him) had written that none of my appointed officials should remain in their responsibility for more than one year; however, you should keep Sayyiduna Abu Musa Ash'ari (may Allah be pleased with him) in his responsibility for four years.