Authentic Supplication Number 186.

When the Messenger of Allah  would see the crescent he used to say: 'Allahu 'akbaru, 'allahumma 'ahillahu `alayna bil-'amni wal 'eemani, was-salamati wal-'islami, wat-tawfeeqi lima tuhibbu watardha, rabbuna wa rabbuka 'allahu (201).

Footnotes.
(201) Allah is greater [than everything], O my ilah bring this month upon us in security and in faith, in peace and Islam [submission], and in making us successful to do what You love and approve of. Our Lord and your Lord is Allah. Shaikh Al-Albani Cautions: Many people face the crescent when supplicating, as they also face graves, and all of this is not allowed as it is affirmed in the legislation of our religion: "Nothing is faced during supplication other than that which is faced during prayer."

Reporters.
Reported by Ad-Daramiy, and At-Tirmithi. Al-Albani said it was sound due to its testimonials and that Ibn Hibban (#2375) said it was sound. At-Tirmithi said it was comely and it is #132 in The Authentic of Good Sayings.