When seeing the new moon
Sighting the crescent
Arabic
اللَّهُمَّ أَهِلَّهُ عَلَيْنَا بِالأَمْنِ وَالإِيمَانِ، وَالسَّلاَمَةِ وَالإِسْلاَمِ
Transliteration
Allāhumma ahillahu ʿalaynā bi-al-amni wa al-īmān, wa as-salāmati wa al-Islām.
English
"O Allah, let it rise over us with security, with faith, with peace, and with Islam."
Source: Sunan at-Tirmidhī 3451
Where the scholars teach this duʿāʾ.
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"Chapter One: On Obligations, Apparent Sunnahs, and Incumbencies in Nullifying It The apparent obligations are six: The first is observing the beginning of the month of Ramadan by sighting the crescent. If it is obscured, then complete thirty days of Sha'ban. By 'sighting,' we mean 'knowing,' which is fulfilled by the statement of one trustworthy person."
— Imam al-Ghazali, Book 6: The Book of Fasting