The Final hadith in the Heart Therapy Series:

The Prophet salla Allahu alayhi wa sallam said: (Call upon Allah while certain of being answered, and know that Allah does not answer a supplication from of a heedless and distracted heart.)
Reported by al-Tirmidhi (660) and al-Hakim (1031), and al-Albani declared it to be hasan (Sahih al-Tirmidhi 3479)

قالَ رسول اللهِ ﷺ: (ادْعُوا اللَّهَ وأنْتُم مُوقِنونَ بالإجابَةِ واعْلَموا أنَّ اللَّهَ لا يَسْتَجيبُ دُعاءً مِنْ قَلْبٍ غَافِلٍ لَاهٍ)
رواه الترمذي (3479) والحاكم (1817) وحسنه الألباني (صحيح الترمذي 3479