Translation of the verse 25 from Surah Ta-Ha : Number of verses 135 - - page 313 - Part 16.
[Musa (Moses)] said: "O my Lord! Open for me my chest (grant me self-confidence, contentment, and boldness).
[Moses] said, "My Lord, expand for me my breast [with assurance]
(20:25) Moses said, "Lord, open my breast for me *14
*14) That is, 'Fill my heart with the courage that may enable me to perform the obligations connected with the great Mission of a Messenger, and give me the confidence for its fulfilment". Prophet Moses prayed for this because he realized the grave responsibilities of the great Mission.
Source : Ta-Ha Verse 25: [Moses] said, "My Lord, expand for me my breast [with assurance]