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I heard a hadith which had a similar meaning to this. Our Holy Prophet (pbuh) criticized a companion for killing a person. The companion had killed the person as he thought that he wasn't a Muslim, although that wasn't the case. Hazrat Mohammad (pbuh) said to the companion that had you seen through his heart whether he was a Muslim or not and that the companion shouldn't have doubted the dead's faith as he had already recited kalma before.

If anyone has any reference to this hadith, kindly mention it in the comments.

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This hadith is reported in Sahih Bukhari:

Allah's Messenger (ﷺ) sent us towards Al-Huruqa, and in the morning we attacked them and defeated them. I and an Ansari man followed a man from among them and when we took him over, he said, "La ilaha illal-Lah." On hearing that, the Ansari man stopped, but I killed him by stabbing him with my spear. When we returned, the Prophet (ﷺ) came to know about that and he said, "O Usama! Did you kill him after he had said "La ilaha ilal-Lah?" I said, "But he said so only to save himself." The Prophet (ﷺ) kept on repeating that so often that I wished I had not embraced Islam before that day. (Sahih Al-Bukhari Version 1 and Version 2)

There are also versions in Sahih Muslim (1 and 2) and Sunan Abi Dawud.

A version in Sahih Muslim and the version of Sunan Abi Dawud mention the phrase about the heart:

He (the Holy Prophet) observed: Did you tear his heart in order to find out whether it had professed or not? And he went on repeating it to me till I wished I had embraced Islam that day.

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