Musnad Ahmad 27:320

Waki’ narrated to us, al-‘Umari narrated to us, from Nafi’, from Ibn ‘Umar: "The Prophet (peace and blessings of Allah be upon him) sent Ibn Rawahah to Khaybar to estimate [the yield of] their crops for them, then he gave them the choice to either take [the estimated amount] or return [it]." They said: "This is the truth; by this, the heavens and the earth were established."