On seeing the new moon
اللَّهُمَّ أَهِلَّهُ عَلَيْنَا بِالْيُمْنِ وَالْإِيمَانِ، وَالسَّلَامَةِ وَالْإِسْلَامِ
O Allah, bring it over us with blessing and faith, safety and Islam.
RamadanJami at-Tirmidhi. Often used at the start of a lunar month, including Ramadan.