Musannaf Ibn Abi Shayba 21:150

Ibn Numayr narrated to us, al-A’mash narrated to us, from Malik ibn al-Harith, who said: Rabi’ used to come to Alqamah. He said: So he came to him, but he was not there. Then a man came and said: "Do you not wonder at the people, the abundance of their supplications, and the scarcity of their being answered?" Rabi’ said: "Do you know why that is? Indeed, Allah does not accept except the pure (al-nakhilah) of supplication. By the One besides whom there is no god, Allah does not hear from one who seeks to be heard (by others), nor one who shows off, nor one who is playing, nor any supplicant except a supplicant who supplicates with firmness from his heart."