On May 17, Twitter user @RDX_AFRIDI showed their concern for the ongoing pre-monsoon floods in Assam by Tweeting a video of a man lying on a floating mattress.

In the tweet, the user wrote "This situation in Assam pls pray/I cannot understand what is going to happen" (translated)

Fact Check

By doing an image reverse search of the keyframes of the video it was found that the video is from Malaysia, shot during the floods at Johor Bahru, Malaysia in January 2021. 
A Malay news website mstar.com published an article on the flood on January 13, 2021.
The man seen in the video is Muhamad Faris Sulaiman, who shared the video on his Instagram profile on January 10, 2021, with the caption “If I’m tired, I can sleep anywhere.”

Sulaiman told mStar that he made the video because his house was flooded and he wanted to vent his frustration. He added that he was happy to be able to entertain others in the face of such a disaster.
In conclusion, the viral video of a man lying on a floating mattress is not from the pre-monsoon Assam floods, but from a flood in Malaysia in January 2021.