Asalaamu alaykum Shaykh. There are two verses that I want to quote if you can please answer this as this is the key to a very good arugment that someone I know is working on. He needs you to explain this and then he is done. Allaah says, {Those remaining behind will say, when you set forth after spoils to acquire them, “Let us follow you,” desiring to change the words of Allaah. Say, “You shall not follow us; thus did Allaah say before.”} (48:15) Allaah says that he said before, but where did he say it, if it is a hadith, then could you please tell me so. Sorry for any inconvenience.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

Allaah said so before they returned from Al-Hudaybiyah, and this did not come down as a Quran to be recited, but it came down in the Sunnah [a revelation to the Prophet but not as Quran].

Tafseer Al-Qurtubi reads: “{Thus did Allaah say before.} [Quran 48:15]; i.e. before we came back from Al-Hudaybiyah, that the booty of Khaybar is exclusively for those who took part in Al-Hudaybiyah. {…they will say: Rather, you envy us.} [Quran 48:15]; i.e. for us to take from the booty just like you. It was said that the Prophet said, 'If you come out, I will not prevent you, but there is no share for you from the booty.' They said, 'This is envy.' The Muslims said, 'Allaah informed us in Al-Hudaybiyah about what they will say, which is the saying of Allaah {…they will say: Rather, you envy us.}, so Allaah says: {But [in fact] they were not understanding except a little.} [Quran 48:15]; meaning that they do not know except the worldly matters.”

Also, At-Tahreer wat-Tanweer (26/168) reads:

“What is meant by the words of Allaah is what He revealed to His Prophet that He promised the people of Al-Hudaybiyah the booty of Khaybar exclusively for them. What is meant here by the words of Allaah is not the Quran, as there was no Quran that was revealed at that time about this. The people of the migration to Abyssinia were joined with them, as the Prophet gave them (from the booty) by revelation.”

Also, As-Saheeh al-Masboor min at-Tafseer bil-Ma’thoor (4/354) reads:

“At-Tabari reported with his authentic chain of narrators from Mujaahid, who said, 'The Prophet returned from Makkah, and Allaah promised him many spoils of war, then the battle of Khaybar took place early; so those who stayed behind said: {"Let us follow you." They wish to change the words of Allaah.} [Quran 48:15], i.e. so that they would take from the spoils of war, about which Allaah said: {When you set out toward the war booty to take it}, and Allaah offered them to fight a people.” 

Allaah knows best.