Sayyiduna Ibn Abbas (may Allah be pleased with them both) said concerning the saying of Allah Almighty: "And remember the favour of Allah upon you and His covenant which He made with you when you said: We hear and we obey; and fear Allah. Indeed, Allah knows well what is within the breasts" [Surah al-Ma'idah: 7]: "That is, when Allah sent the Prophet (peace and blessings be upon him) and revealed the Book to him, they said: We believe in the Prophet (peace and blessings be upon him) and in the Book, and we affirm what is in the Torah. So Allah reminded them of His covenant which they had affirmed upon themselves, and commanded them to fulfil it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12975