Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Indeed, Mu'adh was an ummah (a nation in himself), devoutly obedient to Allah, upright." It was said to him: "O Abu Abd al-Rahman! (The verse is rather) 'Indeed, Ibrahim was an ummah, devoutly obedient to Allah, upright' [Surah al-Nahl: 120]." He said: "Did you hear me mention Ibrahim?" Then he said: "Indeed, Mu'adh was an ummah, devoutly obedient to Allah, upright." He said: "We used to liken Mu'adh to Ibrahim (peace be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9909