Tirmidhi

Hadith Commentary:
The absolute mujtahid, Hazrat Imam Bukhari rahimahullah, wishes to clarify that in the Qur'an and Hadith, the phrase "fi sabilillah" is most often used specifically for jihad.
In the mentioned hadith as well, what is meant is fasting while engaged in jihad, by which voluntary (nafl) fasting is intended, and it is this for which such a virtue is mentioned.
The reality is that the fast of a mujahid (one striving in Allah's cause) and the prayer of a mujahid hold a very lofty status.

Hadith Commentary:
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If there is a fear of any kind of weakness arising from fasting, then for such combatants (mujahideen), not fasting is preferable. This was the practice of Abu Talhah radi Allahu anhu. However, if one is accustomed to fasting and there is no danger of any kind of weakness from it, then fasting while engaging in jihad carries great virtue, because in this there is a combination of two acts of worship.

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The mention of seventy years in the hadith is not for limitation, but rather for emphasis (exaggeration). By this, abundance is intended, meaning that person will be at an unlimited distance from the fire of Hell. This is why, in one narration of Sunan an-Nasa’i, a distance of one hundred years is mentioned. (: Sunan an-Nasa’i, Kitab as-Siyam, Hadith: 2256)

Hadith Commentary: Benefits and Issues: By "fi sabilillah" is meant jihad.

If a mujahid who has set out for jihad is present in the field before the enemy, but fighting is not actually taking place, or he possesses such courage and strength that by fasting he does not show any kind of negligence or weakness in the activities of jihad, then his face—that is, his very self—will, as a result of fasting, be distanced from the fire of Hell by such a long distance, as if, due to the blessing of jihad, his reward will be increased.