Is it permissible for a traveler to combine two prayers before their prescribed time if this is not his usual practice, and has any scholar stipulated this condition?

If the intention of combining prayers before the time [of the second prayer] is to offer the second prayer (Asr) with the first (Dhuhr) as a jama' taqdim (combining by advancing) before the time for the second prayer (Asr) begins, then this is permissible. However, if the intention is to combine both prayers before the time for the first prayer (Dhuhr) begins, then this is not permissible.