Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I used to greet the Prophet (peace and blessings be upon him) with salam and he (peace and blessings be upon him) would reply to me. Once I greeted him (peace and blessings be upon him) and he (peace and blessings be upon him) did not reply. So I said: "I used to greet you and you would reply, but now I greeted you and you did not reply." He (peace and blessings be upon him) said: "Indeed, Allah, the Mighty and Majestic, brings about new matters in His command as He wills."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10499