Letter of Imam 'Ali (a) to Imam al-Hasan (a)
The letter of Imam 'Ali (a) to Imam al-Hasan (a) is a long letter with the ethical and monotheistic content, which he wrote in the last years of his life. Some of important subjects in the letter are: God wariness, supplication, reading history and learning from it, and knowing the time. Also the letter has wise and short sentences. The letter has independent commentaries.
Time and Place of the Writing
The letter is written in the returning of Imam 'Ali (a) from Battle of Siffin. In Nahj al-balagha the place of writing is mentioned as "Hadirin", but it's not clear where the name belongs, or it is not name of a place but is the plural of "hadir" means "people who were present".