Sayyiduna Rafi' bin Khadij (may Allah be pleased with him) says: When the Messenger of Allah (peace and blessings be upon him) saw the new moon, he would say: "This is a new moon of goodness and right guidance." Then he would say: "O Allah, I ask You for the good of it" - three times - "O Allah, I ask You for the good of this month and the good of the decree, and I seek refuge in You from its evil." He would say this supplication three times.
Hadith Referenceالمعجم الكبير للطبراني / باب الراء / 4394