NCBI's FASTA format description:
https://www.ncbi.nlm.nih.gov/genbank/fastaformat/
NCBI's BLAST page describing valid FASTA input:
https://blast.ncbi.nlm.nih.gov/Blast.cgi?CMD=Web&PAGE_TYPE=BlastDocs&DOC_TYPE=BlastHelp
NCBI's SNP page describing FASTA format:
https://www.ncbi.nlm.nih.gov/projects/SNP/snp_legend.cgi?legend=fasta
UniProt's FASTA-header description:
https://www.uniprot.org/help/fasta-headers
Wikipedia reference (indicating format origin):
https://en.wikipedia.org/wiki/FASTA_format
FASTA program (origin of format, per Wikipedia):
https://fasta.bioch.virginia.edu/wrp_fasta/fasta_guide.pdf
Harvard PolyPhen page describing FASTA format: