Dua when sighting the crescent moon
اللهُ أَكْبَر، اللّهُـمَّ أَهِلَّـهُ عَلَيْـنا بِالأمْـنِ...
اللهُ أَكْبَر، اللّهُـمَّ أَهِلَّـهُ عَلَيْـنا بِالأمْـنِ وَالإيمـان، وَالسَّلامَـةِ وَالإِسْلام، وَالتَّـوْفيقِ لِما تُحِـبُّ رَبَّنـا وَتَـرْضى، رَبُّنـا وَرَبُّـكَ الله
Allahu akbar, allahumma ahillahu AAalayna bil-amni wal-eeman, wassalamati wal-islam, wattawfeeqi lima tuhibbu rabbana watarda, rabbuna wa-rabbuka Allah
Translation
Allah is the greatest. O Allah, bring us the new moon with security and faith, with peace and Islam, and in harmony with what our Lord loves and what pleases Him. Our Lord and your Lord is Allah.
When to recite
Recited when sighting the crescent moon at the beginning of a new Islamic month.
Source / Reference
At-Tirmithi 5/504, Ad-Darimi 1/336