Tirmidhi

Explanation:
1:
The virtue of fasting while engaging in jihad applies to those fighters who do not feel weakness as a result of fasting, and those who fear that weakness may overcome them are not included in this virtue.

Note:
(This hadith is also authentic when supported by the next hadith; otherwise, in its chain of narration, "Ibn Lahi'ah" is weak.)

Commentary:
Benefits and Issues:

➊
The mentioned narration has been declared weak in its chain by our esteemed researcher, and he has further written that the previous narration 1717 suffices for this; perhaps for this very reason, other scholars have declared it authentic. Therefore, despite the mentioned narration being weak in its chain, it is still actionable and a valid proof. "In the way of Allah" means to observe it at the time of fighting the disbelievers, provided that there is no likelihood of weakness resulting from it. It can also mean that the fast was observed in order to attain Allah's pleasure, in compliance with His command. Any deed done with sincerity is in the way of Allah.

➌
The meaning of "a distance of seventy years" is that He will distance (the person) from Hell as far as the distance covered in seventy years. It can also mean a very great distance. The distance is likened to a journey of seventy years to clarify the extent of remoteness.

In the hadith, the words ﴿فِي سَبِيلِ اللَّهِ﴾ are mentioned. This applies to every righteous deed that is done purely for the sake of Allah Ta'ala. Since in the Noble Qur'an, ﴿فِي سَبِيلِ اللَّهِ﴾ generally refers to jihad, therefore, the translation can also be: "Whoever fasts during jihad." Furthermore, under ﴿فِي سَبِيلِ اللَّهِ﴾—"in the path of Allah Ta'ala"—fasting during a journey for seeking knowledge, or for Hajj and 'Umrah, etc., is also included. And Allah knows best.