When the moon of Rajab was sighted,
Rasulullah Salallahu Alayhi Wasallam would recite the following Dua :
اللَّهُمَّ بَارِكْ لَنَا فِي رَجَبَ ، وَشَعْبَانَ ، وَبَلِّغْنَا رَمَضَانَ
‘Allahumma baa-rik-lanaa fee-rajaba wa sha’baana wa bal-lig-naa Ramadaan.’
‘O Allah, make the months of Rajab and Shabaan blessed for us,
and let us reach the month of Ramadaan.’ ~ Tabarani