What is the authenticity of the Hadith: "Whoever says: 'May Allah reward Muhammad on our behalf as is befitting for him,' has exhausted seventy angels who write down his reward for a thousand mornings," and what is the meaning of "The Messenger of Allah, peace and blessings be upon him, has spoken the truth"?

The aforementioned hadith is very weak in its chain of transmission through both of its paths, thus it is not authentic from the Prophet, peace and blessings be upon him. As for the saying "The Messenger of Allah, peace and blessings be upon him, has spoken the truth," it is an affirmation of the Prophet's truthfulness, but it is not recommended to always adhere to it after every hadith. When mentioning the hadith, one should say, "Al-Tabarani, may Allah have mercy on him, narrated from Ibn Abbas, may Allah be pleased with both of them, from the Prophet, peace and blessings be upon him."