आहुः सत्यं हि परमं धर्मं धर्मविदो जनाः

Aahuh  satyam hi paramam dharmam dharmavido janaah

  Dasaratha had promised three boons to Kaikeyi when she saved his life during one of those battles he fought on the side of theDevas.  When the coronation of Rama is announced, Kaikeyi asks for two of the boons from Dasaratha.  She wants Bharata to be installed as the King and Rama to be sent to the forest for fourteen years. When Dasaratha was in a dilemma, Kaikeyi says that Truth is the highestdharma, implying that Dasaratha should keep his promise as otherwise he will be guilty of transgressing dharma.