The Prophet ﷺ said, "Fasting is a shield by which the servant protects himself from the Fire. It is for Me, and I Myself reward it."
حدثنا عتاب بن زياد حدثنا عبد الله حدثنا ابن لهيعة حدثني أبو الزبير
عن النبي ﷺ قال إنما الصيام جنة يستجن بها العبد من النار هو لي وأنا أجزي به
Its isnad is sahih, on account of Ibn Lahia and Itaab bin Ziyad Khurasani. Imam Ahmad here is narrating from Itaab, who is one of his youngest students, while Abdullah is Ibn Mubarak, the imam. The hadith has already appeared at 10500.