Musannaf Ibn Abi Shayba 22:23

Abu al-Ahwas narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu al-Ahwas, from ‘Abdullah, who said: Learn the Qur’an and recite it, for Allah rewards you for its recitation with ten good deeds for every letter. As for me, I do not say "Alif-Lam-Mim" [is one letter], but rather "Alif" is a letter, "Lam" is a letter, and "Mim" is a letter.