Bahirah, the Monk Recognizes the Seal of Prophethood

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 tolearn this from our books."
Source: At Tirmidhi and At Tabari