Reference: Fataawa Ramadhaan – Volume 2, Pages 931-932, Fatwa No.914
Majmoo’ Fataawa Samaahatu ash-Shaykh ‘Abdul Azeez Ibn ‘Abdullaah Ibn Baaz – Volume 3, Page 101
Question: What is the ruling regarding the one who had not discharged the Zakaatul-Fitr until during the [‘Eed] khutbah after the ‘Eed prayer, and that is because he had forgotten?
Response: Discharging the Zakaatul-Fitr before the [‘Eed] prayer is obligatory, and whoever forgets then there is nothing upon him except to discharge it after that. This is because it is an obligation so it is upon him to discharge it when he remembers, and it is not permissible for anyone to intentionally delay it until after the ‘Eed prayer according to the most correct of the two opinions. This is because the Messenger (sal-Allaahu ‘alayhi wa sallam) commanded the Muslims to discharge it before the ‘Eed prayer.