In the hadith of Sayyiduna Abu Hurayrah (may Allah be pleased with him) about the incident of the man who entered (the mosque) and prayed, and then said to the Prophet (peace and blessings be upon him): "By the One who sent you with the truth! I cannot pray any better than this, so teach me." So he (peace and blessings be upon him) said: "When you stand for prayer, say the takbir." Then he mentioned the complete hadith.
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