Sahih Bukhari Volume 9, Book 93, Hadith Number 547.

Narrated By Ibn ‘Abbas : The Prophet said, “O Gabriel, what prevents you. from visiting us more often than you do?” Then this Verse was revealed: ‘And we angels descend not but by Command of your Lord. To Him belongs what is before us and what is behind us…’ (19.64) So this was the answer to Muhammad.