You're looking for a PDF version of "Rasa'il al-Junaid" (also known as "Letters of Junaid").
Academic Archives: Look for versions edited by scholars like Ali Hassan Abdel-Kader, who famously compiled and translated these works into English. kitab rasail al-junaid pdf
Throughout the book, al-Junaid offers numerous spiritual insights, including: You're looking for a PDF version of "Rasa'il