On the authority of Ka‘b ibn ‘Āsim, who said: I heard the Messenger of Allah (peace and blessings be upon him) say: “It is not part of righteousness to fast while traveling.”(1)

2256 – Ibrāhīm ibn Ya‘qūb informed us, saying: Muḥammad ibn Kathīr narrated to us, from al-Awzā‘ī, from al-Zuhrī, from Sa‘īd ibn al-Musayyib, who said: The Messenger of Allah (peace and blessings be upon him) said: “It is not part of righteousness to fast while traveling.”(2) Abū ‘Abd al-Raḥmān said: This is an error, and the correct version is the one preceding it; we know of no one who followed Ibn Kathīr in this.

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(1) Its chain of transmission is sound. Sufyān: he is Ibn ‘Uyaynah. Al-Zuhrī: he is Muḥammad ibn Muslim ibn Shihāb. Umm al-Dardā’: she is the younger one, the wife of Abū al-Dardā’, and her name is Hujaymah, and it is also said: Juhaymah. It is in “al-Sunan al-Kubrā” under number (2575).
It was also recorded by Aḥmad (23681) and Ibn Mājah (1664) via the chain of Sufyān ibn ‘Uyaynah, with this chain of transmission.
It was also recorded by Aḥmad (23680) via the chain of Ibn Jurayj, from al-Zuhrī, with it.
See the following narration.
Al-Sindī said: His statement “It is not part of righteousness…” etc., with kasrah on the bā’, meaning: of obedience and worship. Its apparent meaning is that refraining from fasting is more appropriate, since fasting is prescribed as an act of obedience; so if it departs from being an act of obedience, then it should not be permissible—or at the very least, refraining is more appropriate. Those who say that fasting is more appropriate while traveling apply the ḥadīth to its specific context, i.e., it is not part of righteousness when the fasting person reaches such a degree of hardship. It is as if this is based on defining “fasting” as referring to a specific instance, and pointing to the fasting of that particular person. Indeed, the principle is the generality of the wording, not the specificity of the context—but when the generality of the wording leads to a conflict between evidences, it is interpreted according to the specificity of the context, as is the case here. It was also said: “min” in his statement “laysa min al-birr” is redundant, and the meaning is: it is not righteousness itself, rather breaking the fast may be more righteous than it when one is on a pilgrimage or jihad, in order to gain strength for it. In sum, the meaning is one of restriction, due to the definiteness of both terms. It was also said: the ḥadīth is to be interpreted as referring to one who fasts and does not accept the concession.
(2) A sound ḥadīth, but this chain of transmission is weak due to its discontinuity, and the compiler has declared it defective immediately after it. Al-Awzā‘ī: he is ‘Abd al-Raḥmān ibn ‘Amr. It is in “al-Sunan al-Kubrā” under number (2576).
It has already preceded in the one before it.