Musannaf Ibn Abi Shayba 22:21

Marwan ibn Mu'awiyah narrated to us, from 'Abd al-Malik ibn Abjar, from al-Minhal ibn 'Amr, from Qays ibn Sakan, who said: 'Abdullah said: "Learn the Qur'an, for indeed, for every letter of it, ten good deeds are written, and ten evil deeds are expiated by it. As for me, I do not say 'Alif-Lam-Mim' [is one letter], but I say: 'Alif' is ten, 'Lam' is ten, and 'Mim' is ten."