Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
Bukhari :: Book 6 :: Volume 60 :: Hadith 255
Narrated Ibn Abbas:
The Prophet said to Gabriel, "What prevents you from visiting us more often than you visit us now?" So there was revealed:--
'And we (angels) descend not but by the command of your Lord. To Him belongs what is before us and what is behind us...'(19.64)