Musnad Ahmad Hadith 22061

It is narrated from Mu'adh (may Allah be pleased with him) that when the Noble Prophet (ﷺ) sent him towards Yemen, he asked him, "If a decision comes before you, how will you resolve it?" He replied, "I will decide it in the light of the Book of Allah." The Noble Prophet (ﷺ) asked, "If you do not find it in the Book of Allah, then what will you do?" He replied, "Then I will decide in the light of the Sunnah of the Noble Prophet (ﷺ)." The Noble Prophet (ﷺ) asked, "If you do not find its ruling in my Sunnah either, then what will you do?" He replied, "Then I will exercise ijtihad with my own opinion." Upon this, the Noble Prophet (ﷺ) said, "All praise is due to Allah, Who has granted the messenger of His Messenger the ability to do that which pleases His Messenger."