From the time of amud hashachar, it is prohibited to deal with one’s work before praying. This is because holy matters precede secular matters and the respect of Heaven precedes the needs of people. Therefore, it is necessary to first thank Hashem in prayer, and only afterwards deal with one’s own needs. The Chachamim teach (Berachot 14a), “Anyone who prays and afterwards goes on his way, HaKadosh Baruch Hu grants him what he desires.”
It is preferable to pray individually before beginning to work, instead of praying in a minyan after he has begun to work. For example, if a person must start work at 6:30 a.m. and the only minyan in the area is at 7:30 a.m., it is better that he prays individually before work begins, so that he does not work prior to praying (Mishnah Berurah 89:20).
However, before amud hashachar, a person is permitted to deal with his work, for since the time of Shacharit has not yet arrived, he is not considered to be putting his needs before prayer. He must be strict in saying Birkot HaShachar before that, for the time to recite them is immediately upon waking up. Since he started his work before the time to pray began, he is permitted to continue even after amud hashachar on condition that he succeeds in praying before the time to pray lapses(ShulchanAruch 89:7; Mishnah Berurah 89:37; 70:23).
. The Rambam (Hilchot Tefillah 6:7) writes that it is permissible to get a haircut and enter a bathhouse close to the time of Shacharit, meaning before amud hashachar. This is because Chazal only prohibited these activities close to Minchah time for that is when people normally perform such activities. That is how the Shulchan Aruch 89:7 rules. However, the Ra’avad maintains that this prohibition also applies within the half hour before amud hashachar, as explained in Bei’ur Halachah 70:5. Some say that the Rambam is lenient only concerning bathing and haircuts, for those activities are not common before Shacharit, but regarding activities that are more commonly performed before amud hashachar, he is stringent (Pri Megadim and Derech HaChaim). Most poskim are lenient concerning all activities. The Mishnah Berurah 89:37 slightly tends toward the opinion of those who are stringent and therefore writes that one must recite Birkot HaShachar before performing such activities, because after reciting Birkot HaShachar there are poskim who are lenient, as cited by the Rama 89:3.
Regarding a person who begins an activity after amud hashachar has already arrived, since he started when he was forbidden to do so, he must stop to recite Keriat Shema, for it is a biblical obligation. However, for the Amidah he does not need to interrupt; he can finish what he started on condition that he will succeed in praying on time (Mishnah Berurah 70:23).