Seven Surahs of the Qur’an begin with the letters ح م and are called al-Hawamin. They are the XL, XLI, XLII, XLIII, XLIV, XLV, and XLVI. Various opinions are held by Muslim commentators as to the meaning of these mysterious letters. Jalalu ‘d-din as-Suyuti in his Itqan, says these letters are simply initial letters, the meaning of which is known only to God, but Ibn ‘Abbas says the letters ح, and م stand for الرحمان ar-Rahman, “the Merciful,” one of the attributes of God.
Mr. Rodwell, in his Introduction to the Koran, says, “Possibly the letters Ha, Mim, which are prefixed to numerous successive Suras were private marks, or initial letters, attached by their proprietor to the copies furnished to Said when effecting his recession of the text under Othman. In the same way, the letters prefixed to other Suras may be monograms, or abbreviations, or initial letters of the names of the persons to whom the copies of the respective Suras belonged.”
Based on Hughes, Dictionary of Islam