Surah Yunus, Ayat 104

Ayat 104. Say (O Muhammad  [SAWW](PBUH): "O you mankind! If you are in doubt as to my religion (Islam), then (know that) I will never worship those whom you worship, besides Allah. But I worship Allah Who causes you to die, I am commanded to be one of the believers. 

Hazrat Talhah bin Ubaidullah (May Allah be pleased with him) reported: At the sight of the new crescent (moon of the lunar month), the Messenger of Allah  [SAWW](PBUH) used to supplicate: "Allahumma ahillahu `alaina bil-amni wal-iman, was-salamati wal-Islam, Rabbi wa Rabbuk-Allah, Hilalu rushdin wa khairin (O Allah, let this moon appear on us with security and Iman; with safety and Islam. (O moon!) Your Rubb and mine is Allah. May this moon be bringing guidance and good).'' [At-Tirmidhi Hadith # 3451].  

Lesson : as mentioned above in Surah Yunus Ayat 104. "I am commanded to be one of the believers." The sign of True believer is to always remember Allah, After sighting the moon, one should recite the prayer which was recited by the Prophet  [SAWW](PBUH). The prayer cited in the text of this Hadith is one of them.