Bismillahi Al-Rahmani Al-Rahim
§ It is not permissible for anyone who is required to pray Jumu`ah to travel after its time enters[fn1]. Al-Shafi`i, Ishaq and Ibn Mundhar had this opinion. Abu Hanifah held that it was permissible. Al-Awza`i was asked about a traveler who hears the adhan of Jumu`ah right after he has reading his mount; he answered that he continues with his journey since `Omar (Allah be pleased with him) said that Jumu`ah does not restrain one from traveling.
But our evidence is what Ibn `Omar (Allah be pleased
with them both) related that the Messenger of Allah (Allah bless him
and give him peace) said, "Whoever travels from a place of residence
on Jumu`ah: the angels make du`a against him that he is not
accompanied nor assisted in his need," [which] Al-Daraqutni related
in Al-Afrad, and this threat would not be attached to
something permissible. And [also] because Jumu`ah has become
obligatory for him [to perform] so it is not permissible for
him engage in anything that prevents him from [the Jumu`ah prayer],
such as entertainment and commerce. And [as for] what was related
from `Omar (Allah be pleased with him), narrations been related from
his son and from `A'ishah (Allah be pleased with them and their
parents) indicating the offensiveness of traveling on Friday, so
[these narrations] oppose his so we interpret them to apply to
traveling before the zenith.
The first [opinion] is it being prevented [unlawful], because of the hadith of `Omar (Allah be pleased with him) ^.
The second [opinion] is it being permissible, and it is the opinion of Hasan [Al-Basri], Ibn Sirin and the majority of scholars. [This is] because of the hadith of `Omar (Allah be pleased with him) ^, and since Jumu`ah [prayer of that particular day] has not become obligatory it is not unlawful to travel, just as during the night.
The third [opinion] is that it is permissible [to travel] for jihad but not for other things, and this is what Al-Qadi [Abu Ya`la] mentioned. [This is] because of what Ibn `Abbas (Allah be pleasd with him) related, that
the Prophet (Allah bless him and give him peace) sent Zaid bin Haritha, Ja`far bin Abi Talib and `Abd Allah bin Rawahah in the army to Mu'ta. `Abd Allah (Allah be pleased with him) stayed back and the Prophet (Allah bless him and give him peace) saw him and asked him why he stayed behind. "Jumu`ah," he replied. So the Prophet (Allah bless him and give him peace) said, "An errand for the sake of Allah at night (or: early morning) is better than this world and all that it contains." And so he quickly departed.
Ahmad related it in Al-Musnad. [...]
This is what Ibn Qudamah writes in Al-Mughni
(2:228-9, Dar Al-Kutub Al-`Ilmiyyah edition. Beginning with: wa
man tajib `alayhi al-jumu`ah la yajuz lahu al-safr ba`d dukhul
It is well reported that Imam Ahmad (may Allah be pleased with him) said that if he does not have evidence for a specific issue he would go with whatever Imam Al-Shafi`i (may Allah be pleased with him) said. One explanation that can be given to Imam Ahmad's statement is that what this action of his is what Allah Most High has ordered us to do:
O ye who believe! Obey Allah, and obey the Messenger and those of you who are in authority; and if you have a dispute concerning any matter, refer it to Allah the Messenger if you are true believers in Allah and the Last Day. That is beneficial and of better consequences. (Qur'an 4:59)
and So ask the people of the Reminder if you do not know (Qur'an 21:7).
And Allah knows best.
[Sources: Al-Mughni (2:228-9), Mughni
Al-Muhtaj (1:278-9), Tuhfat Al-Muhtaj (1:491-2),
Kifayat Al-Akhyar, Al-Raud Al-Murbi`, Al-Salsabil Fi Ma`rifat
Al-Dalil, Al-Wajiz Fi Usul Al-Fiqh, The Majestic