What is the ruling on someone who established the prayer (iqamah) before the imam in the Asr prayer and completed three rak'ahs, then completed the fourth rak'ah with the imam when he established the prayer and the people aligned themselves?

Scholars have differed regarding the ruling on the prayer of a person praying alone who intends to join the congregational prayer during his salat. Some say it is invalidated due to the division of his intention, while others say it is not invalidated. Sheikh Ibn Uthaymeen affirmed the view that it is not invalidated, but it is safer to repeat the prayer.