Mashrafe said this during the toss prior to the first of two T20Is against Sri Lanka at R Premadasa Stadium in Colombo. The two-match T20I series against the Lankans will be Mashrafe's last assignment before he reserves all his attention for ODIs.
The announcement also came in his official facebook page minutes before the toss ceremony.
In the facebook post, Mashrafe said, "It has been a great honour for me to represent Bangladesh in T20Is for more than 10 years. I believe that this team are currently a balanced one and there are some promising youngsters. I am very much grateful to BCB and the high officials for keeping the trust on me to lead this fantastic team.
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Bangladesh cricket captain Mashrafe Mortaza celebrates after Bangladesh defeated Sri Lanka by 90 runs in the first one day international (ODI) cricket match between Sri Lanka and Bangladesh at The Rangiri Dambulla International Cricket Stadium in Dambulla on March 25, 2017 AFP
"I am also immensely grateful to my loving fans, family, and friends for supporting me all the time. There were some bad and good days as well. I always tried to make my fans happy. I am very sorry to each and every fan for not being able to make them happy in every match. Right now as a team we are playing good cricket. I am sure Bangladesh will keep on playing good cricket in the days to come.
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Bangladesh's Mashrafe Mortaza bowls during the second one-day international cricket match between New Zealand and Bangladesh at the Saxton Oval in Nelson on December 29, 2016 AFP
This is high time for me to say goodbye to the T20I format so that youngsters can get the opportunity to showcase their talent and thus, the BCB can nurture them for the future. Let me congratulate the new captain in advance and I am sure the best is yet to come.
See you soon, love!"