The second one is the standard.
The letter ह messes with the phonetic pronunciation of a lot of words (such as यह, बहन, महल etc). When the consonant ह is present between two अ-vowels, both these vowels take an 'e' sound. For example, in पहला, ह् is between the अ attached to प and the one attached to ह so both these अs are pronounced as 'e'. In the absence of this vowel change, the second अ in this word would not have been pronounced so the second 'e' sound is sometimes omitted. As a result, पहला is pronounced 'pehela' or 'pehla'.
This 'e'-sound is formally absent in Hindi but is the shorter form of the ए vowel (or sometimes, the shorter version of ऐ).