It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that some people from Yemen came to the Messenger of Allah (peace and blessings be upon him) and said: "Send with us a man who will teach us the Sunnah and Islam." He (peace and blessings be upon him) took Sayyiduna Abu Ubaydah bin al-Jarrah (may Allah be pleased with him) by the hand and said: "This is the trustworthy one (amin) of this Ummah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2268