Arabic text
عن النعمان بن بَشِير رضي الله عنه قال: سمعت النبيَّ صلى الله عليه وسلم يقول: «الدُّعَاءُ هُوَ الْعِبَادَةُ»، ثُمَّ قَرَأَ: «{وَقَالَ رَبُّكُمُ ادْعُونِي أَسْتَجِبْ لَكُمْ إِنَّ الَّذِينَ يَسْتَكْبِرُونَ عَنْ عِبَادَتِي سَيَدْخُلُونَ جَهَنَّمَ دَاخِرِينَ} [غافر: 60]».
Translation
An-Nu‘mān ibn Bashīr (may Allah be pleased with him) reported: I heard the Prophet (may Allah's peace and blessings be upon him) say: "Supplication is worship." Then, he recited: {Your Lord says: "Call upon Me; I will respond to you. Those who are too proud to worship Me will enter Hell, utterly debased."} [Surat Ghāfir: 60]
Explanation
The Prophet (may Allah's peace and blessings be upon him) informed that supplication is worship. So, it should be purely directed to Allah, whether supplication is that of request, as the supplicant asks Allah Almighty to bring him what is beneficial and avert from him what is harmful in this world and in the Hereafter, or supplication of worship, involving such words and deeds, inward and outward, that are beloved and pleasing to Allah, acts of worship performed within the heart or through one's body or money. Then, the Prophet (may Allah's peace and blessings be upon him) cited as evidence the verse that reads: {Call upon Me; I will respond to you. Those who are too proud to worship Me will enter Hell, utterly debased.}
Source: HadeethEnc.com