The Messenger of Allah ﷺ commanded us to give the charity of Fitr before the obligatory zakat was revealed. When zakat was revealed, he neither commanded us nor forbade us from giving it, and we continued to do so.
حدثنا علي بن محمد حدثنا وكيع عن سفيان عن سلمة بن كهيل عن القاسم بن مخيمرة عن أبي عمار
أمرنا رسول الله ﷺ بصدقة الفطر قبل أن تنزل الزكاة فلما نزلت الزكاة لم يأمرنا ولم ينهنا ونحن نفعله
Sourcing, [sahih] Narrated by Nasai, 49/5, Zakat, chapter on the obligation of Sadaqat al-Fitr before the revelation of zakat, hadith no. 2509, from the hadith of Waki with the same wording. Hakim graded it sahih according to the criteria of the two Shaykhs, 410/1, and Dhahabi agreed with him. Thawri narrated it using the "an" formula, but Shubah corroborated him in Mushkil Athar by Tahawi, 85/3. The hadith has another sahih route with Nasai and others. It was Shubah's habit not to narrate from narrators known for concealment in transmission except what they explicitly stated they had heard directly.