Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: We used to greet the Prophet (peace and blessings be upon him) with salam during the prayer, and he would return our greeting. Then one day I greeted him, and he did not respond, so I asked him about it, and he (peace and blessings be upon him) said: "Allah, the Mighty and Majestic, brings about whatever new ruling He wills from His command."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10093