What is the ruling on the widespread saying, "Behold, the righteous have long yearned for My meeting..."? And is he who speaks it and preaches to people with it sinful, knowing that it is not authentic?

This statement is not an authentic Hadith, but its meanings are good. If it is mentioned as an admonition and not as a Hadith, there is no harm in that. A part of it has been mentioned in the words of verifying scholars such as Ibn Taymiyyah and Ibn al-Qayyim, and there is no objection to reminding people of it, provided it is clarified that it is not authentic as a Hadith.