Hadhrat Ubaadah bin Saamit (Radi Allahu Anhu) narrates that Rasulullah (Sallallahu Alayhi Wasallam) used to teach the Sahaabah (Radi Allahu Anhum) the following Dua at the approach of the Month of Ramadhaan:
اَللَّهُمَّ سَلِّمْنِيْ لِرَمَضَانَ وَسَلِّمْ رَمَضَانَ لِيْ وَسَلِّمْهُ لِيْ مُتَقَبَّلاً
Allahumma Sallimnee Liramadaana Wa Sallim Ramadaana Lee Wa Sallimhu li Mutaqabbila
“O Allah! Safeguard me for the Month of Ramadhaan (by making me see the Month of Ramadhaan healthy and fit so that I can take maximum benefit from it), and safeguard the Month of Ramadhaan for me (by making the conditions in it such that I can take maximum benefit from it) and accept it from me.” (Kanz-ul-ummal, Vol.8, Pg. 584 Hadith 24277)
Every Muslim should endeavor to memorize this dua and recite it in abundance in these few days before Ramadhaan.