When the Prophet of Allah ﷺ reached Khaybar at dawn, its people had already taken up their tools and gone out to their fields and lands. When they saw the Prophet of Allah ﷺ with the army, they turned back in retreat.The Prophet of Allah ﷺ said, "Allah is the Greatest, Allah is the Greatest. When we descend upon the territory of a people, evil is the morning of those who were warned."
لما صبح نبي الله ﷺ خيبر، وقد أخذوا مساحيهم وغدوا إلى حروثهم وأرضيهم. فلما رأوا نبي الله ﷺ ومعه الجيش نكصوا مدبرين فقال نبي الله ﷺ: الله أكبر الله أكبر، إنا إذا نزلنا بساحة قوم فساء صباح المنذرين.
Sahih. Ahmad (16347) and Tabarani in Al-Kabir (5/99–100) both narrated it from the hadith of Saeed bin Abu Aruba, from Qatada, from Anas, from Abu Talha, and he mentioned it. Its chain of transmission is sahih. Saeed bin Abu Aruba did become confused in his later years, but Rawh bin Ubada narrated from him in Ahmad's version before this confusion set in. This hadith may belong to the musnad of Abu Talha, but Anas bin Malik would sometimes narrate it from him, and sometimes narrate it as part of his own musnad, because he too had taken part in the fighting.