Bahá'u'lláh departed from Tihran for Iraq on 12 January 1853. Among those who accompanied Him in His exile was His eldest son, nine-year-old Abbas, who later assumed the title of Abdu'l-Bahá (Servant of Baha). He had such spiritual insight that, as a young boy, He intuitively recognized the station of His Father. So highly did Bahá'u'lláh esteem Him that in Baghdad He used to address Him, while still in His teens, as the Master -- a designation which Bahá'u'lláh had also used for His own father while in Tihran.
(Adib Taherzadeh, The Revelation of Baha'u'llah v 1)