The Messenger of Allah ﷺ said, "Whoever fasts a single day for the sake of Allah, Allah will distance his face from the Fire by seventy autumns for that day."
أن رسول الله ﷺ قال: من صام يوما في سبيل الله، زحزح الله وجهه عن النار بذلك سبعين خريفا.
Nasai (2244) and Ahmad (7990) both narrated it from the hadith of Anas bin Iyad, from Suhayl bin Abu Salih, from his father, from Abu Hurairah, and he mentioned it. This isnad is sahih.The hadith has other chains as well, including what Ibn Majah (1718) narrated from Hisham bin Ammar, who said, Anas bin Iyad narrated to us, who said, Abdullah bin Abdul-Aziz Al-Laythi narrated to us, from Maqburi, from Abu Hurairah, who mentioned the hadith in similar fashion. Hisham bin Ammar Al-Sulami Al-Dimashqi may have erred in attributing this to Anas bin Iyad, because when he grew old he began to be prompted by others. As for Abdullah bin Abdul-Aziz Al-Laythi, Ibn Hibban said of him, "Toward the end of his life he became confused and would mix up the chains of transmission."Another chain is what Tirmidhi (1622) narrated from Qutayba, who said, Ibn Lahia narrated to us, from Abu Aswad, from Urwa and Sulayman bin Yasar, who both narrated to him from Abu Hurairah, from the Prophet ﷺ, and he mentioned it similarly. He said, one of them says "seventy" and the other says "forty." Tirmidhi said, "This is a gharib hadith from this angle, and Abu Aswad's name is Muhammad bin Abdul-Rahman bin Nawfal Al-Asadi Al-Madani." End of quote. In the isnad is Ibn Lahia, about whom there is well known discussion, and some scholars held the view that Qutayba bin Saeed was among those who heard from him even before his confusion set in.