ENOfficial
He reported God’s messenger as saying, “Supplicate God when you are assured of being answered, and know that God does not answer a supplication which comes from a careless and inattentive heart.” Tirmidhi transmitted it, saying this is a gharib tradition.
Source: sunnah.com
Other translations
Arabic
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «ادْعُوا اللَّهَ وَأَنْتُمْ مُوقِنُونَ بِالْإِجَابَةِ وَاعْلَمُوا أَنَّ اللَّهَ لَا يَسْتَجِيبُ دُعَاءً مِنْ قَلْبٍ غَافِلٍ لَاهٍ» . رَوَاهُ التِّرْمِذِيُّ وَقَالَ: هَذَا حديثٌ غَرِيب
sunnah.com