Yahya related to me from Malik that Zayd ibn Aslam used to say,"No-one makes a dua without one of three things happening. Either itis answered, or it is stored up for him, or wrong actions are atonedfor by it." (PT)
USC-MSA web (English) reference: Book 15, Hadith 36 (PT)
Arabic reference (PT): Book 15, Hadith 508 (PT)