Tafsir on the Verse:
"O those who believe, if a sinful person brings you a report, verify its correctness, lest you harm a people out of ignorance then become remorseful on what you did."
Background of Revelation
According to Ibn Kathir, it is reported in Musnad of Ahmad that Harith Ibn Dirar Ibn Abi Dirar, the leader of the tribe of Banul-Mustaliq, whose daughter Sayyidah Juwairiyah (RA) was one of the blessed wives of the Holy Prophet (saw), came up to the Holy Prophet (saw), and he called him towards Islam and asked him to pay his Zakah. He says "I embraced Islam and pledged to pay Zakah, and said to the Holy Prophet (saw) that I would go to my people, call them towards Islam and ask them to pay Zakah. Those who would accept my invitation and pay the compulsary alms, I would collect their alms. I requested that on a designated date of a designated month, a collector be sent to me, so that the alms might be handed over to him". Accordingly, when Harith had collected the alms of the believers and the alms collector did not arrive on the designated date or even long after that, he feared that may be the Holy Prophet (saw) may be displeased with them for some reason or the other. Otherwise it was unlikely for the Holy Prophet (saw) not to keep his promise. Harith expressed his apprehension to the leaders of the new Muslims, and wanted them to go to the Holy Prophet (saw). On the other hand, the Holy Prophet (saw) had sent Walid Ibn Uqbah to collect the Zakah from them. However, on his way he thought that the members of that tribe are his old enemies and feared that they might kill him. With this fear he returned to Madinah, and said to the Holy Prophet (saw) that they refused to pay Zakah, and wanted to kill him. On hearing this, the Holy Prophet (saw) became very angry, and under the command of Khalid ibn Walid (ra) he dispatched a force of fighters in the way of Allah. On the one hand this contingent was dispatched, and on the other hand Harith Ibn Dirar with his people was on his way to meet the Holy Prophet (saw). The two groups met near Madinah. Harith inquired: "To whom have you been sent?" They replied: "We are sent to you". Harith inquired: "For what purpose?" They narrated the mission of Walid ibn Uqbah who reported to the Holy Prophet (saw) that the tribe of Banul-Mustaliq refused to pay Zakah and planned to kill him. Harith said an oath, "I swear by Allah who sent Muhammed as His true Messenger, Walid did not arrive in the location, nor did I see him. Since no envoy had come to us at the appointed location, I apprehended that you are annoyed with me due to some shortcoming on my part. Therefore, I have come here. Harith says that the present verse of Surah Al-Hujurat was revealed on this occasion (Ibn Kathir).
Other versions have it that Walid Ibn Uqbah did go to the tribe of Banul-Mustaliq. As the tribe was expecting the Holy Prophet's (saw) envoy to arrive on a designated date, they came out of their settlement as a mark of respect to welcome him. Walid Ibn Uqbah suspected that they might have come out to kill him, on account of their old hostility. Therefore he returned instantly from there, and went up to the Holy Prophet (saw) and reported to him according to his suspicion, that they were not willing to pay the alms and wanted to kill him. On hearing this report the Holy Prophet (saw) dispatched Sayyidna Khalid Ibn Walid (ra) to make a throrough investigation, so that appropriate measures could be taken. Sayyidna Khalid Ibn Walid (ra) arrived near the settlement at night and encamped there. He selected a few of his men and sent them furtively as spies into the location to investigate. They returned and reported that the members of the tribe were Muslims and believers; they were regular with their prayers and in paying their alms; and they did not find the tribe doing anything contrary to Islam. Sayyidna Khalid Ibn Walid (ra) came back, and reported the whole story to the Holy Prophet (saw). This verse was revealed on that occasion (Ibn Kathir - summary of several reports).
On the basis of this verse, it may be ruled that if a mischief-monger, sinner, wicked or corrupt person complains about any person or nation, or accuses them of any wrong-doing, it is not lawful to act upon the sole information or evidence of such a person without making a thorough investigation.
Injunctions and rulings related to verse 
Imam Jassas in Ahkam-ul-Quran says that this verse indicates that it is not lawful to accept any information conveyed by a sinful person and act upon it unless it is investigated by other sources and confirmed. This verse contains the injuction fatabayyanu (verify its correctness). In another reading the word "tathabbatu" conveys the same meaning. The sense is: "Do not rush into taking actions or measures; be stable, and stand firmly in your place; and do not be easily moved or shaken. Wait until the news or information is confirmed by other independent sources. Since it is not lawful to accept the information or report of an ungodly sinful person, then the testimony of such a person would not be acceptable with greater force, because every testimony is a statement which is confirmed by a kind of oath.
Therefore, according to majority of scholars, a statement or evidence of an ungodly person, in terms of Shari'ah, is not acceptable. However all scholars agree that as far as the common worldly affairs are concerned, it is not necessary to ascertain the accuracy or truth of every news and the reliability of every informer. The jurists are clear that this rule of law does not apply in the case of ordinary matters, because the rule is governed by an effective cause which is laid down in the explicit text of this verse (lest you harm a people out of ignorance 49:6). Thus matters which are not governed by the effective cause will be an exception to, and excluded from, the rule of this verse. For instance, if a wicked person, or for that matter even an unbeliever, delivers a gift to someone, stating that this gift has been sent by a certain person, it would be lawful to accept the gift and the statement.