Sayyiduna Anas Ibn Malik (radiyallahu'anhu) reports that Rasulullah (sallallahu ‘alayhi wa sallam) would recite the following supplication when the Month of Rajab would commence:
اَللّٰهُمَّ بَارِكْ لَناَ فِيْ رَجَبَ وَشَعْبانَ وَبَلّغْنَا رَمَضَانْ
Allahumma barik lana fi Rajaba wa Sha'bana wa balligh na Ramadan
Translation: Oh Allah! Grant us Barakah (Blessing) during (the months of) Rajab and Sha'ban, and allow us to reach Ramadan.