Najmul guides Bangladesh to comfortable win against Sri Lanka
Najmul hit 122 not out off 129 balls, and was ably supported by one of his predecessors as captain, Mushfiqur Rahim, who was unbeaten on 73, as Bangladesh raced to 257-4 in 44.4 overs after bowling out Sri Lanka for 255 runs.
The pair added 165 in their unbroken fifth-wicket stand, saving Bangladesh from a potential mid-innings collapse, coming together at 92-4.
Dilshan Madushanka took two early wickets and Pramod Madushan joined him to leave Bangladesh struggling at 23-3, in a little over five overs.
Najmul put on 69 for the fourth wicket with Mahmudullah Riyad to stave off the initial danger.
Lahiru Kumara broke the partnership by removing Mahmudullah for 37, but Najmul and Mushfiqur denied Sri Lanka further success.
Najmul cut Kumara for a boundary to reach his third ODI century off 108 balls, and finished the game with a four off Maheesh Theekshana.
"We knew that with the new ball, it's quite hard, so I just... took a little bit of time," Najmul said.
"Later on, the wicket was very easy because of the dew."'Struggled'
Earlier, Tanzim Hasan Sakib, Taskin Ahmed and Shoriful Islam claimed three wickets each to restrict the Sri Lankan batsmen.
Janith Liyanage top-scored with 67 off 69 balls, after Kusal Mendis made 59 off 75 balls in Sri Lanka's roller-coaster innings after they elected to bat first.
The visitors got off to a rollicking start with openers Pathum Nissanka (36) and Avishka Fernando (33) adding 71 in less than 10 overs.
Tanzim claimed three wickets in quick succession to slow the flow of runs, finishing with 3-44.
Mendis and Charith Asalanka (18) put on 44 runs for the fourth wicket to prevent a complete collapse.
Liyanage joined Mendis after Mehidy Hasan Miraz dismissed Asalanka.
Mendis lamented the tourists' batting in the middle overs.
"Pathum and Avishka batted really well in the first 10 overs," he said. "The middle order struggled a little bit after that. I think 280 plus would have been a better score on this surface."
Taskin ended the promising innings of Mendis, forcing the Sri Lankan captain to give a catch to his Bangladesh counterpart Najmul at mid-off.
Taskin, who finished with 3-60, dismissed Wanindu Hasaranga and Theekshana, but Liyanage ensured Sri Lanka still got to a decent score.
Shoriful (3-51) removed Liyanage, before wrapping up Sri Lanka's innings in 48.5 overs.
The visitors won the preceding T20 series 2-1.
It was the first ODI between the teams since the World Cup last year in India, when Sri Lanka veteran Angelo Mathews became the first player in the 146-year history of international cricket to be dismissed timed out.
The two sides meet again on Friday, and wrap up the series on Monday.