At what point is a traveler permitted to commence shortening the prayers

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Check below answers in case you are looking for other related questions:
Is it permissible for one who is in prison to shorten his prayers and put them together?
He was traveling but he forgot and stood up for a third rak’ah

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Related Answers:
Shortening prayers when travelling for work
The traveller has to offer the prayer in full if he prays behind an imam who is offering it in full
If a traveller decides to return part way through his journey, can he still shorten his prayers on his way back?
Is it permissible for one who is in prison to shorten his prayers and put them together?
He was traveling but he forgot and stood up for a third rak’ah

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