Praise be to Allah.
It was narrated by Abu Dawood (3635), at-Tirmidhi (1940), Ibn Maajah (2342), Ahmad (15755), at-Tabaraani in al-Kabeer (830), al-Bayhaqi in his Sunan (11386), al-Kharaa’iti in Masaawi’ al-Akhlaaq (583) from Abu Sirmah, that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever harms [others], Allah will harm him, and whoever causes hardship [to others] Allah will cause hardship to him.”
At-Tirmidhi said, after quoting this report: This is a hasan ghareeb hadith. Al-Albaani also classed it as hasan in Saheeh at-Tirmidhi. It was also classed as hasan by the commentators on al-Musnad. Ibn Muflih said in al-Aadaab ash-Shar‘iyyah (1/11): Its isnaad is jayyid [good].
There is corroborating evidence in the hadith of Abu Sa‘eed al-Khudri which was narrated by al-Haakim (2345) and al-Bayhaqi (11384), which says: “There should be neither causing harm nor reciprocating harm. Whoever harms [others], Allah will harm him, and whoever causes hardship [to others], Allah will cause hardship to him.”
Al-Qaari (may Allah have mercy on him) said:
“Whoever harms [others]” means harming a believer, and being the first to stir up trouble.
“Allah will harm him” means: He will requite him for his action, by way of making the punishment fit the crime.
“Whoever causes hardship” means by opposing someone and showing hostility towards him,
“Allah will cause hardship to him” means He will punish him.
End quote from Mirqaat al-Mafaateeh (8/3156).
The questioner says “but people are harming others at present, and Allah is not harming them”; this may be answered in several ways:
There is nothing in the hadith to suggest that the requital for this harm must be in this world; rather the basic principle regarding requital is that it will take place in the hereafter, which is the arena of requital, although Allah may hasten whatever requital He wills to whomever He wills of His slaves. Allah, may He be exalted, says (interpretation of the meaning):
“And never think that Allah is unaware of what the wrongdoers do. He only delays them for a Day when eyes will stare [in horror]”
As-San‘aani (may Allah have mercy on him) said:
“Whoever harms” means: whoever causes harm to a Muslim or a mu‘aahid [a non-Muslim living under Muslim rule], or in fact to any living being whose life is protected [under Islamic law], for no just cause.
“Allah will harm him” that is, He will send upon him great harm in this world and the hereafter, or in one of them.
“Whoever causes hardship [to others]” for no just cause, “Allah will cause hardship to him” as a requital for what he did. Allah has forbidden people to harm others and cause hardship to others; rather He has enjoined upon them the opposite of that. Therefore the best of people are those who are best to others, and the most beloved of people to Allah are those who bring the most benefit to His slaves.
End quote from at-Tanweer Sharh al-Jaami‘ as-Sagheer (10/298).
Muslim (2581) narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Do you know what bankrupt means?” They said: Among us, the one who is bankrupt among us is the one who has no money or wealth. He said: “The one who is bankrupt among my ummah is the one who will come on the Day of Resurrection with prayer, fasting and zakaah, but he will come having insulted this one, slandered that one, consumed the wealth of this one, shed the blood of that one and beaten this one. They will each be given from his good deeds, and if his good deeds run out before the scores have been settled, some of their bad deeds will be taken and cast upon him, then he will be thrown into Hell.”
Hastening of punishment in this world, like the punishment of sin for Muslims in the hereafter, is a matter that is subject to the will of Allah, may He be glorified and exalted. If He wills, He will punish a person for that in this world; or if He wills, He will delay his punishment until the hereafter; or if He wills, He will pardon the person and will compensate the one who was wronged for what he suffered of harm with something that will please him, if the one who caused harm and hardship repents.
Harm may befall a person in this world at the end of his life, so he is sent back to a feeble age, his children defiantly disobey him, his wife leaves him, his wealth is lost and the like. So today you see him well off, but Allah will make things difficult for him and cause harm to befall him at the end of his life. But people are hasty in seeking vengeance against those who have wronged them, and if they see such a person in a state of well-being today, they think that he will remain like that all his life.
We see something similar in the reports about the prayer of one who has been wronged, and how Allah supports him against the one who wronged him.
Ahmad narrated in al-Musnad (8043), from Abu Hurayrah (may Allah be pleased with him) in a marfoo‘ hadith:
There are three whose supplication will not be rejected: a just ruler; a fasting person until he breaks his fast; and the supplication of one who has been wronged, which is carried on the clouds, and the gates of the heavens are opened for it, and the Lord, may He be glorified and exalted, says: By My might, I shall surely support you, even if it is after a while.
The commentators on al-Musnad said: a saheeh hadith because of its isnaads and corroborating evidence. End quote.
And Allah knows best.