He sent a hadeeth in order to spread good, then he found out that it is a fabricated hadeeth. What should he do?

Praise be to Allah.

Spreading da’wah messages and letters which contain explanations of shar’i rulings, or preaching and beneficial stories, is an important means of doing good, because many people receive them, and it is easy to send them. But we should make sure that the content is sound and that the ahaadeeth contained in it are saheeh (sound), because some people are abusing this great blessing (of easy communication) and are spreading fabricated ahaadeeth and false stories. 

It is not permissible for anyone to say, “The Messenger of Allaah (peace and blessings of Allaah be upon him) said…” when he knows that the hadeeth is mawdoo’ (fabricated) or false. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever tells lies about me deliberately, let him take his place in Hell.” Narrated by al-Bukhaari, 1291; Muslim, 933. 

And he (peace and blessings of Allaah be upon him) said: “Whoever narrated a hadeeth from me which he thinks is false, then he is one of the liars.” Narrated by Muslim in the Introduction to his Saheeh. 

Al-Nawawi (may Allaah have mercy on him) said in Sharh Muslim (1/71): 

It is haraam to narrate a fabricated hadeeth for one who knows that it is fabricated or who thinks it most likely that it is fabricated. Whoever narrates a hadeeth which he knows or thinks it most likely to be fabricated, and does not explain that it is fabricated, is included in this warning and is included among those who tell lies about the Messenger of Allaah (peace and blessings of Allaah be upon him). This is also indicated by the hadeeth quoted above: “Whoever narrated a hadeeth from me which he thinks is false, then he is one of the liars.” Hence the scholars said that the one who wants to narrate or quote a hadeeth must check it. If it is saheeh or hasan, then he may say, “The Messenger of Allaah (peace and blessings of Allaah be upon him) said such and such, or did such and such, etc”, expressing certainty. If it is da’eef (weak), then he should not say, “he said” or “he did” or “he commanded” or “he forbade” and other such definitive words, rather he should say, “Such and such was narrated from him” etc, or “it was narrated” or “it was said” or “we heard that” and so on. And Allaah, may He be glorified, knows best. End quote. 

He also said: 

With regard to the prohibition on telling lies about the Prophet (peace and blessings of Allaah be upon him), no distinction is made between reports which contain rulings and those which do not contain rulings, such as preaching and exhortation, etc. In all cases it is haraam and is a major sin and abhorrent action, according to the consensus of the Muslims whose consensus counts, unlike the innovators who falsely claim that it is permissible for fabricate ahaadeeth of exhortation, and who are followed in that by many of the ignorant who claim to be ascetics. End quote. 

With regard to what you have done by spreading this fabricated hadeeth, the expiation for that is to ask Allaah for forgiveness and to strive to inform everyone to whom you think you most likely sent this hadeeth that the hadeeth is fabricated. And Allaah burdens not a person beyond his scope [al-Baqarah 2:286]. 

You should not spread any hadeeth after this until you make sure that they are soundly attributed to the Messenger of Allaah (peace and blessings of Allaah be upon him). 

And Allaah knows best.