Shaykh Taqiyudeen (Shaykhul Islam Ibn Taymiyah) said: 
“From the manners of Ad Dua is the praise upon Allah and The Salat upon his messenger Sallallahu Alaihi Wassallam, Dua is from the greatest means of bringing about benefits and repelling the harms, it is recommended to conceal the dua for it is greater in submissiveness and closer to sincerity”
Tawdeehul Ahkaam volume 1 Page 618
Majid Jawed Al Afghaanee