The boiling point of sulfuric acid compared to phosphoric acid is a matter of established chemical fact and everyday textbook knowledge, requiring no citation.
The claim compares the physical boiling points of two common inorganic acids, which are standard tabular data found in chemical handbooks (sulfuric acid decomposes near its boiling point around 337 °C, while phosphoric acid melts around 42 °C and boils/decomposes at higher temperatures or lower pressures, but sulfuric acid's boiling point characteristics are extremely well-known). This falls under common knowledge, requiring no specific literature receipt.