To what extent is the poem of Ibn al-Mubarak, "O worshipper of the Two Holy Mosques," authentic, what is its chain of transmission (sanad), where can it be found in the accredited references, what are the conditions of the men in its chain of transmission in terms of disparagement and affirmation (jarh wa ta'deel), and which contemporary scholars have authenticated it?

The poem you referred to is well-known and famous among scholars, and many of them have mentioned it in their books, such as Al-Hafiz Ibn Asakir, Ibn Kathir, and Al-Dhahabi. The latter stated that Ibn al-Mubarak dictated the poem and sent it with Muhammad ibn Ibrahim to Fudayl ibn Iyad, who wept upon reading it and said: "Abu Abd al-Rahman spoke the truth and offered good counsel." Denying this story entirely is unreasonable, especially since the early Muslims (Salaf) were not strict in narrating biographies, reports, and stories, as Al-Iraqi said in his Alfiyyah concerning the Sirah (Prophet's biography): "Let the seeker know that the Sirah gathers what is authentic and what has been denied."