Put into bat, left-handed Sayan struck his third consecutive half-century scoring a well-compiled 87 as he added 140 runs for the second wicket with another in-form batsman Sudeep Chatterjee (65).
76 overs of play was possible on the day due to fading light which is a common phenomenon in the eastern part of India.
It was off-break bowler Gokul Sharma, whose straighter found Chatterjee's pads.
Sayan, who had by then crawled towards his maiden three figure mark saw part-time leggie Amit Verma bowl one straight and was caught plumb in-front.
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Skipper Manoj Tiwary was then caught by verma for 12 off seamer Krishna Das, who had earlier accounted for opener Abhimanyu Easwaran (8) in his first spell.
Brief Scores: Bengal 1st innings 191/4 (Sayan sekhar Mondal 87, Sudeep Chatterjee 65, Krishna Das 2/39) vs Assam.