Is the ḥadīth that states: "A river sweeter than honey, whiter than milk." They said: "For whom, O Messenger of Allah?" He said: "For whoever hears my name and invokes blessings upon me," authentic?

This Hadith has no basis from the Prophet, peace and blessings be upon him, and it is not permissible to attribute it to him. Whoever narrates a Hadith from him, knowing it to be false, is one of the liars. What is close to it has been narrated from Ali ibn Abi Talib, may Allah be pleased with him, with a different wording. There is also a Hadith about the virtue of fasting narrated by Al-Tabarani, which has a weak chain of transmission. This is sufficient, making one independent of fabricated and weak Hadiths regarding the virtue of praying upon the Prophet, peace and blessings be upon him.