This is an anecdote from the early Islamic conquests, specifically the Battle of Ullais, also known as the "Day of the Lion." This battle took place in 633 AD during the Muslim conquests of Persia.

According to historical accounts, during this battle, the Persian forces released a lion to intimidate the Muslim army. However, the lion was reportedly killed by a prominent Muslim warrior. There are various versions of the story, and it is often said that the warrior who killed the lion was Khalid ibn al-Walid, one of the most famous generals in early Islamic history. Khalid ibn al-Walid, known as "The Sword of Allah," played a crucial role in the early Muslim conquests and was known for his bravery and military acumen.

While this story has been passed down through generations and is widely known in Islamic tradition, the specifics of such events can sometimes be embellished or altered in the retelling. Nevertheless, the tale of the lion at the Battle of Ullais remains a significant part of the narrative of early Islamic military history.

For more detailed accounts, you can refer to historical texts such as:

1. **"The History of al-Tabari" (Tarikh al-Tabari)**: This multi-volume historical work includes a detailed account of early Islamic history and the conquests.
2. **"The Conquest of Iraq, Southwestern Persia, and Egypt: Volume XI" by al-Tabari**: This specific volume covers the events related to the Muslim conquests of these regions, including the Battle of Ullais.

These sources provide a rich context for understanding the events and figures of early Islamic conquests.