Al-Bayhaqi reported: Al-Shafi’i, may Allah be pleased with him, said, “You must acquire understanding in the religion before assuming leadership. If you assume leadership first, there will be no way to acquire understanding.”
Source: al-Madkhal ilā al-Sunan al-Kubrá 271
عن البيهقي قال الشافعي رَضِيَ اللَّهُ عَنْهُ تَفَقَّهْ قَبْلَ أَنْ تَرَأَّسَ فَإِذَا تَرَأَّسْتَ فَلا سَبِيلَ إِلَى التَّفَقُّهِ
271 المدخل إلى السنن الكبرى للبيهقي باب فضل العلم