ISLAMABAD, Pakistan, Sept. 4—The treason trial of Sheik Mujibur Rahman, the Bengali nationalist leader, has been adjourned to allow the defense time to prepare its case, a highly placed source reported today.

Sheik Mujib was arrested by the Pakistan Army at his home in Dacca, East Pakistan, on March 26, on charges, among others, of “waging war against Pakistan.” Last December Sheik Mujib's Awami League party had won a majority in the National Assembly, in East Pakistan's first free election.

The Pakistani Government had announced that the trial had begun Aug. 11, before military tribunal. The proceedings are secret.

Sources here say that Pakistan intends to continue barring outside observers and there are indications that the trial may last several inure Weeks.