Get it here:
and download hg18.2bit, this is hg18 binary coded. This one you can convert to the fasta format using twoBitToFa. You can download this tool here:
Ok, then you can download the single chromosomes here:
Skip that ones with random in the name, just from chr1.fa.gz to chrY.fa.gz (depends on what you need - may ask the guy who did the alignment).
After that unzip them and merge. How this works please find here:
fa equals fasta
The BAM file will not contain the reference genome (if that is what you are asking). Check the header:
samtools view -H my.bam
You may find some information about the exact version that was used to align the data. If you can't find anything I'd suggest you contact the person that generated the alignments.