Sunan Abi Dawud 1479
سنن أبي داود#1479SahihNarrated 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
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 8:64