He formed the intention to do sa‘i during the first lap
Question

A man went to the Mas‘aa (place of sa‘i) and started walking, thinking that it was the place for tawaaf, then when he reached the green marker, he realised that he was in the Mas‘aa, so he formed the intention to do sa‘i. What is the ruling on that?
Praise be to Allah.He must go back to the beginning, and if he did not do that, then let him repeat sa‘i now, because the first sa‘i was not valid, as he did not have the intention of doing it from the beginning, and he formed the intention whilst doing it. So now he must repeat sa‘i. End quote.
Majmoo‘ Fataawa Ibn ‘Uthaymeen (24/100)