Al-Matalib al-'Aliyah 3343 — The virtue of supplication

It is narrated from Imam al-Hasan al-Basri (may Allah have mercy on him) that he used to say: "Sometimes Allah, the Mighty and Majestic, delays the answering of a servant's supplication and grants him its reward on the Day of Resurrection, because He does not like that, in exchange for it, the servant should receive some merely temporary and passing benefit of this world."
Imam al-Harith recorded this narration in his Musnad (Bughyat al-Bahith); in its chain, al-Rabi' bin Subayh is a saduq (truthful) narrator, but there is some criticism of his memory. However, the meaning of supplication being answered and stored up for the Hereafter is established by other sahih hadith.