Sunan Abi Dawud 1479
سنن أبي داود#1479Sahihحَدَّثَنَا حَفْصُ بْنُ عُمَرَ، حَدَّثَنَا شُعْبَةُ، عَنْ مَنْصُورٍ، عَنْ ذَرٍّ، عَنْ يُسَيْعٍ الْحَضْرَمِيِّ، عَنِ النُّعْمَانِ بْنِ بَشِيرٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " الدُّعَاءُ هُوَ الْعِبَادَةُ { قَالَ رَبُّكُمُ ادْعُونِي أَسْتَجِبْ لَكُمْ } " .
Narrated An-Nu'man ibn Bashir: The Prophet (ﷺ) said: Supplication (du'a') is itself the worship. (He then recited:) "And your Lord said: Call on Me, I will answer you" (xI)
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 8:64