The Prophet ﷺ said, "Supplication is worship itself." Then he recited, "Your Lord has said, 'Call upon Me, and I will answer you. Those who are too proud to worship Me will enter Hell humiliated.'" [Ghafir 40:60].
عن النبي ﷺ قال: الدعاء هو العبادة ثم قرأ {وقال ربكم ادعوني أستجب لكم إن الذين يستكبرون عن عبادتي سيدخلون جهنم داخرين}[سورة غافر: ٦٠].
Reported by Abu Dawud (1479), Tirmidhi (3372), Ibn Majah (3828), and Ahmad (18352), and graded sahih by Ibn Hibban (890) and Hakim (1/490-491), all of them through various routes from Dhirr bin Abdullah Hamdani, from Yasi Hadrami, from Numan bin Bashir, who related it. Its chain is sahih. Tirmidhi said, "This is a hasan sahih hadith." Hakim said, "Its chain is sahih."Hakim also reported (1/491), grading it sahih, through two chains from Ibn Abbas that he said, "The best form of worship is supplication," and he recited, "Your Lord has said, 'Call upon Me and I will answer you. Indeed, those who are too arrogant to worship Me will enter Hell in humiliation.'" [Ghafir 40:60]As for what has been reported from Anas bin Malik that the Prophet ﷺ said, "Supplication is the marrow of worship," this is daif. It was reported by Tirmidhi (3371), and Tabarani in al-Awsat (3220) and in al-Duaa (8), both through the route of Ibn Lahia, from Ubaydullah bin Abu Jafar, from Abu Aban bin Salih, from Anas bin Malik, who related it. Tirmidhi said, "It is unfamiliar in this form, we know it only through the hadith of Ibn Lahia." Ibn Lahia had poor memory, and he erred in this hadith by saying, "Supplication is the marrow of worship." The correct wording is, "Supplication is worship itself," for supplication has no marrow, rather supplication in its entirety is worship.