Musannaf Ibn Abi Shayba 33:809

Ya'la ibn 'Ubayd narrated to us, from Mis'ar, from 'Atiyyah, from Abu Sa'id, who said: When the servant says, "Praise be to Allah abundantly" (Al-hamdu lillahi kathiran), the angel says, "How shall I write it?" He (Allah) says, "Write for him My mercy abundantly." And when he says, "Allah is the Greatest, abundantly" (Allahu akbaru kabiran), the angel says, "How shall I write it?" He (Allah) says, "Write for him My mercy abundantly."