What is the authenticity of the hadith: "There are no two servants who love each other for the sake of Allah, but when one of them meets his companion, shakes his hand, and they both invoke blessings upon the Prophet (peace and blessings be upon him), they will not part until their sins, past and future, are forgiven"? And should one act upon it if it is weak?

The hadith (definition) mentioned was deemed weak by Sheikh Al-Albani, and it is: "There are no two servants who love each other for the sake of Allah, and one greets the other, shakes his hand, and they both invoke blessings upon the Prophet - peace and blessings be upon him - except that they do not part until their sins, past and present, are forgiven." Many authentic hadiths have been narrated regarding the virtue of shaking hands, among them: "There are no two Muslims who meet and shake hands, except that they are forgiven before they part," and "If two Muslims meet, shake hands, praise Allah - Exalted be He - and seek His forgiveness, they will be forgiven." There is no objection to acting upon a weak hadith in matters of virtuous deeds, as it falls under the general principles that encourage remembrance (dhikr) and invoking blessings upon the Prophet - peace and blessings be upon him.