ENOfficial
Ibn ‘Umar reported God’s messenger as saying, "Supplication is beneficial regarding things which have happened and things which have not happened, so devote yourselves to supplication, servants of God." Tirmidhi transmitted it, and Ahmad transmitted it from Mu'adh b. Jabal. Tirmidhi said this is a gharib tradition.
Source: sunnah.com
Other translations
Arabic
وَعَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «إِنَّ الدُّعَاءَ يَنْفَعُ مِمَّا نَزَلَ وَمِمَّا لَمْ يَنْزِلْ فَعَلَيْكُمْ عِبَادَ اللَّهِ بِالدُّعَاءِ» . رَوَاهُ التِّرْمِذِيُّ وَرَوَاهُ أَحْمَدُ عَنْ مُعَاذِ بْنِ جَبَلٍ. وَقَالَ التِّرْمِذِيّ هَذَا حَدِيث غَرِيب
sunnah.com