The Invasion of Dul Ashir occurred directly after the Invasion of Buwat in December of 623 AD (2 A.H of the Islamic calendar). The expedition was ordered by Muhammad after he received intelligence that a caravan was heading to Syria. Therefore, Muhammad directed about 150-200 Muslims, who attempted to raid the caravan.But caravan left before the Muslims arrived.
They had thirty camels that they rode upon by turns. When they arrived at al-Usharayh, they expected to raid a rich Meccan caravan towards Syria led by Abu Sufyan. Muhammad already had the knowledge of this caravan’s departure from Mecca and waited for about a month for this caravan to pass. But the Meccan caravan had already passed.
In this operation, Muhammad entered into an alliance with Banu Madlaj, a tribe inhabiting the vicinity of al-Ushayra. He also concluded another treaty that was made with Banu Damrah previously.
Modern secondary sources which mention this, include the award winning book,Ar-Raheeq Al-Makhtum (The Sealed Nectar)