What one day of fasting is measured against
The unit given is not a distance.
مَنْ صَامَ يَوْمًا فِي سَبِيلِ اللَّهِ تَبَارَكَ وَتَعَالَى بَاعَدَ اللَّهُ وَجْهَهُ عَنِ النَّارِ سَبْعِينَ خَرِيفًا
Man ṣāma yawman fī sabīli-Llāhi tabāraka wa taʿālā bāʿada-Llāhu wajhahu ʿani-n-nāri sabʿīna kharīfā
“Whoever fasts one day in the path of Allah, Allah puts seventy autumns between his face and the Fire.”
Where this comes from
Kharīf is the autumn, the harvest season, the way a farmer counts a year. Seventy of them, bought with one day of not eating.
The full narration
Abu Saeed Al-Khudi said:"I heard the Messenger of Allah say: 'Whoever fasts one day in the cause of Allah, Allah will separate his face from the Fire by (a distance of) seventy autumns
Source
Sunan an-Nasā'ī · 2250
Ṣaḥīḥ
- Abu Ghuddah: Sahih
- Al-Albani: Sahih
- Zubair Ali Zai: Hasan