اَللَّهُمَّ أَهْلِلْهُ عَلَيْنَا بِالْيُمْنِ وَالْإِيْمَانِ وَالسَّلَامَةِ وَالْإِسْلَامِ رَبِّيْ وَرَبُّكَ اللَّهُ

Allaahumma ahlilhu `alaina bil-yumni wal-iman, was-salamati wal-Islam, rabbi wa rabbuk Allah

O Allah, bring it over us with blessing and faith, and security and Islam. My Lord and your Lord is Allah. Rasulullah (ﷺ) used to say this dua when he saw the new moon. It is masnun to recite this dua after seeing the new moon of Ramadan and all the months.

Reference: Sahih. Tirmidhi: 3451