Dear sir I have buy a stock on Monday and it automatically sell after closing NSE.

I am not understand. There is any rule document for trading. Please give me a link.

Who is your broker?

Many brokers square-off open positions at the end of the day.

While buying the shares, you have to specify if you are buying for intra-day trading or delivery.

If you buy for intra-day trading, your broker will sell the shares at the end of the day.

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I also have the same situation.