When his Lord said to him, "Submit," he said, "I have submitted [in Islām]1 to the Lord of the worlds."
Translation: Saheeh International (English)
1 The meaning of the word "Islām" is "submission to the will of Allāh." This is the way of life ordained by Allāh and taught by all of the prophets from Adam to Muḥammad (ﷺ). A Muslim is one who submits himself to Allāh.
Allah chose Abraham because of his swiftness in surrendering, telling him to be faithful and devoted to Him in worship and to humbly do as He instructed. Abraham replied to his Lord saying that he had surrendered in devotion to Him, Who created His servants, providing for them and taking care of their affairs.