When the Messenger of Allah (Peace be upon him) was twelve years old, he went with his uncle Abu Talib on a business journey to Syria. When they reached Busra (which was a part of Syria, in the vicinity of Howran under the Roman domain) they met a monk called Bahira (his real name was Georges), who showed great kindness, and entertained them lavishly. He had never been in the habit of receiving or entertaining them before. He readily enough recognized the Prophet (Peace be upon him) and said while taking his hand: "This is the master of all humans. Allah will send him with a Message which will be a mercy to all beings." Abu Talib asked: "How do you know that?" He replied: "When you appeared from the direction of 'Aqabah, all stones and trees prostrated themselves, which they never do except for a Prophet. I can recognize him also by the seal of Prophethood which is below his shoulder, like an apple. We have got to learn this from our books."
Source: At Tirmidhi and At Tabari