The signature of God's Oneness is found in all the sacred texts, in blessed revelations across the lands. The gift of surrendering to The One, The Only One, is ever here, now, inviting us in each moment to awaken.
Judaism "Hear Oh Israel, The Lord our God, the Lord is One." (Deuteronomy 6:4). "I am the LORD, and there is no other; apart from me there is no God." (Isaiah 45:5) "I am the Alpha and I am the Omega; apart from me there is no God." (Isaiah 46:6).
Christianity "Jesus answered, 'The most important [commandment] is: Hear Oh Israel, the Lord Our God is One.'" (Mark 12:29). "Therefore, as to the eating of food offered to idols, we know that an idol has no real existence, and that there is no God but One.” (1 Corinthians 8:4). "A mediator, however, implies more than one, but God is One." (Galatians 3:20). "Why do you call me good?" Jesus replied. "No one is good, except God alone." (Mark 10:18).
Islam "God: there is no God but He..." (Quran 2:255). "...God is the Creator of all things, and He is the One..." (Quran 13:16). "Say: He—God—is One..." (Quran 112:1).
Hinduism "Do not worship anyone besides Him alone; praise Him alone." (Rig Veda, Book 8, Hymn 1, Verse 1). "He is One only without a second." (Upanishads). "He is One, and one remains alone. There is no second God." (Atharva Veda). "God is One, but we can approach him in many ways." (Rig Veda).
Sikhism "Seek the Support of the One Lord, and surrender your soul to Him; place your hopes only in the Sustainer of the world." (The Adi Granth).
Baha'i "Bear thou witness in thine inmost heart unto this testimony which God hath Himself and for Himself pronounced, that there is none other God but Him, that all else besides Him have been created by His behest, have been fashioned by His leave, are subject to His law, are as a thing forgotten when compared to the glorious evidences of His oneness, and are as nothing when brought face to face with the mighty revelations of His unity." – Baha'u'llah
Barbra Streisand! Okay, okay, she is not a messenger, but this enlightening song, One God, sung by Barbra Streisand says it all.