اَللَّهُمَّ بَارِكْ لَنَا فِي رَمَضَانَ وَتَقَبَّلْ مِنَّا الصِّيَامَ وَالْقِيَامَ
Transliteration
Allahumma barik lana fi Ramadana wa taqabbal minnas-siyama wal-qiyam
English meaning
O Allah, bless Ramadan for us, and accept from us our fasting and night prayers.
When is it recited?
At the start of Ramadan and daily.
Source
Traditional Ramadan supplication